485BPOS

AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON DECEMBER 26, 2012

FILE NO.  333-160918

FILE NO.  811-22321

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933  [X]

Post-Effective Amendment No.  36

AND

REGISTRATION STATEMENT

UNDER THE INVESTMENT COMPANY ACT OF 1940  [X]

Amendment No.  38

MAINSTAY FUNDS TRUST

(exact name of registrant as specified in charter)

51 MADISON AVENUE, NEW YORK, NEW YORK 10010

(address of principal executive office)

REGISTRANT'S TELEPHONE NUMBER: (212) 576-7000

J. Kevin Gao, Esq. MainStay Funds Trust 169 Lackawanna Avenue Parsippany, NJ 07054 Copy to: Sander M. Bieber, Esq. Dechert LLP 1775 I Street, NW Washington, DC 20006

(NAME AND ADDRESS OF AGENT FOR SERVICE)

It is proposed that this filing will become effective

x Immediately upon filing pursuant to paragraph (b) of Rule 485

o on ________ pursuant to paragraph (b)(1) of Rule 485

o 60 days after filing pursuant to paragraph (a)(1) of Rule 485

o on ________, pursuant to paragraph (a)(1) of Rule 485

o 75 days after filing pursuant to paragraph (a)(2) of Rule 485

o on  ________  pursuant to paragraph (a)(2) of Rule 485

If appropriate, check the following box:

o This Post-Effective Amendment designates a new effective date for a previously filed post-effective amendment

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Registration Statement under Rule 485(b) under the Securities Act and that it has duly caused this Post-Effective Amendment No. 36 to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Parsippany in the State of New Jersey, on the 26^th day of December, 2012.

MAINSTAY FUNDS TRUST
By:	/s/ Stephen P. Fisher
	Stephen P. Fisher
	President and Principal Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 36 to the Registration Statement has been signed below by the following persons in the capacities indicated on December 26, 2012.

SIGNATURE	TITLE
/s/ Stephen P. Fisher	President and Principal Executive Officer
Stephen P. Fisher
/s/ Susan B. Kerley*	Trustee and Chairman of the Board
Susan B. Kerley
/s/ John Y. Kim*	Trustee
John Y. Kim
/s/ Alan R. Latshaw*	Trustee
Alan R. Latshaw
/s/ Peter Meenan*	Trustee
Peter Meenan
/s/ Richard H. Nolan, Jr.*	Trustee
Richard H. Nolan, Jr.
/s/ Richard S. Trutanic*	Trustee
Richard S. Trutanic
/s/ Roman L. Weil*	Trustee
Roman L. Weil
/s/ John A. Weisser*	Trustee
John A. Weisser
/s/ Jack R. Benintende	Treasurer and Principal Financial
Jack R. Benintende	and Accounting Officer
*By: /s/ J. Kevin Gao
J. Kevin Gao
As Attorney-in-Fact

*    PURSUANT TO POWERS OF ATTORNEY PREVIOUSLY FILED

EXHIBIT INDEX

EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund

MainStay Short Duration High Yield Fund

Investment Objective

The Fund seeks high current income.

Capital appreciation is a secondary objective.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you may pay if you buy and
hold shares of the Fund. You may qualify for sales charge discounts if you and
your family invest, or agree to invest in the future, at least $50,000 in the
MainStay Funds. This amount may vary depending on the MainStay Fund in which
you invest. More information about these and other discounts is available from
your financial professional and in the "Information on Sales Charges" section
starting on page 20 of the Prospectus and in the "Alternative Sales Arrangements"
section on page 76 of the Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees MainStay Short Duration High Yield Fund	Investor Class		Class A		Class C	Class I	Class R2
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	3.00%		3.00%		none	none	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	none	[1]	none	[1]	1.00%	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses MainStay Short Duration High Yield Fund		Investor Class	Class A	Class C	Class I	Class R2
Management Fees (as an annual percentage of the Fund's average daily net assets)		0.65%	0.65%	0.65%	0.65%	0.65%
Distribution and/or Service (12b-1) Fees		0.25%	0.25%	1.00%	none	0.25%
Other Expenses	[1]	0.53%	0.48%	0.53%	0.48%	0.58%
Total Annual Fund Operating Expenses	[2]	1.43%	1.38%	2.18%	1.13%	1.48%
Waivers / Reimbursements	[2]	(0.33%)	(0.33%)	(0.33%)	(0.33%)	(0.33%)
Total Annual Fund Operating Expenses After Waivers / Reimbursements	[2]	1.10%	1.05%	1.85%	0.80%	1.15%

Example

The Example is intended to help you compare the cost of investing in the Fund
with the cost of investing in other mutual funds. The Example assumes that you
invest $10,000 in the Fund for the time periods indicated and then redeem all
of your shares at the end of those periods. The Example also assumes that your
investment has a 5% return each year and that the Fund's operating expenses
remain the same.

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example MainStay Short Duration High Yield Fund (USD $)	Investor Class	Class A	Class C	Class I	Class R2
Expense Example, with Redemption, 1 Year	409	404	288	82	117
Expense Example, with Redemption, 3 Years	707	692	650	326	436
Expense Example, with Redemption, 5 Years	1,028	1,002	1,139	590	777
Expense Example, with Redemption, 10 Years	1,934	1,880	2,488	1,345	1,740

Expense Example, No Redemption (USD $)	MainStay Short Duration High Yield Fund Class C
Expense Example, No Redemption, 1 Year	188
Expense Example, No Redemption, 3 Years	650
Expense Example, No Redemption, 5 Years	1,139
Expense Example, No Redemption, 10 Years	2,488

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells
securities (or "turns over" its portfolio). A higher portfolio turnover may
indicate higher transaction costs and may result in higher taxes when Fund
shares are held in a taxable account. These costs, which are not reflected in
annual fund operating expenses or in the Example, affect the Fund's performance.
Because the Fund has been in operation for less than one full calendar year, the
Fund's portfolio turnover rate for the most recent fiscal year is not available.

Principal Investment Strategies

The Fund, under normal circumstances, invests at least 80% of its assets
(net assets plus any borrowings for investment purposes) in high-yield debt
securities that are rated below investment grade by an independent rating
agency, such as Standard & Poor's ("S&P"), Moody's Investor  Service, Inc.
("Moody's") or Fitch Ratings ("Fitch"), or that are unrated but are
considered to be of comparable quality by MacKay Shields LLC, the Fund's
Subadvisor. Debt securities in which the Fund may invest include all types
of debt obligations such as bonds, debentures, notes, bank debt, bank loan
participations, commercial paper, floating rate loans, U.S. Government
securities (including obligations, such as repurchase agreements, secured
by such instruments), and convertible corporate bonds. The Fund will generally
seek to maintain a weighted average duration of three years or less, although
the Fund may invest in instruments of any duration or maturity. Duration is a
measure used to determine the sensitivity of a security's price to changes in
interest rates. The longer a security's duration, the more sensitive it will be
to changes in interest rates.

Securities that are rated below investment grade by independent rating agencies
are commonly referred to as "junk bonds." These securities are sometimes
considered speculative. If independent rating agencies assign different ratings
to the same security, the Fund will use the lower rating for purposes of
determining the security's credit quality.

The Fund may invest up to 20% of its net assets in equity securities, including
preferred shares. The Fund also may invest in securities of non-U.S. issuers.
The Fund may hold cash or invest in investment grade short-term instruments
during times when the Subadvisor is unable to identify attractive high-yield
securities.

In times of unusual or adverse market, economic or political conditions, the
Fund may invest without limit in investment grade securities and may invest
in U.S. government securities or other high quality money market instruments.
Periods of unusual or adverse market, economic or political conditions may
exist in some cases, for up to a year. To the extent the Fund is invested in
cash, investment grade debt or other high quality instruments, the yield on
these investments tends to be lower than the yield on other investments normally
purchased by the Fund. Although investing heavily in these investments may help
to preserve the Fund's assets, it may not be consistent with the Fund's primary
investment objective and may limit the Fund's ability to achieve a high level of
income.

Investment Process: The Subadvisor seeks to identify investment opportunities
through analyzing individual companies and evaluates each company's competitive
position, financial condition, and business prospects. The Fund seeks to
minimize interest rate risk through its emphasis on duration management and
investments in securities with short and intermediate maturities. The Fund only
invests in companies in which the Subadvisor has judged that there is sufficient
asset coverage-that is, the Subadvisor's subjective appraisal of a company's
value divided by the value of its debt, with the intent of maximizing
default-adjusted income and returns.

The Subadvisor may sell a security if it no longer believes the security will
contribute to meeting the investment objective of the Fund. In considering
whether to sell a security, the Subadvisor may evaluate, among other things,
meaningful changes in the issuer's financial condition and competitiveness.

Principal Risks

Loss of Money Risk: Before considering an investment in the Fund, you should
understand that you could lose money.

Net Asset Value Risk: The Fund is not a money market fund, does not attempt to
maintain a stable net asset value, and is not subject to the rules that govern
the quality, maturity, liquidity and other features of securities that money
market funds may purchase. Under normal conditions, the Fund's investment may be
more susceptible than a money market fund to interest rate risk, valuation risk,
credit risk and other risks relevant to the Fund's investments. The Fund's net
asset value per share will fluctuate.

Market Changes Risk: The value of the Fund's investments may change because of
broad changes in the markets in which the Fund invests, which could cause the
Fund to underperform other funds with similar objectives. From time to time,
markets may experience periods of acute stress that may result in increased
volatility. Such market conditions tend to add significantly to the risk of
short-term volatility in the net asset value of the Fund's shares.

Liquidity and Valuation Risk: Securities purchased by the Fund that are liquid
at the time of purchase may subsequently become illiquid due to events relating
to the issuer of the securities, market events, economic conditions or investor
perceptions. The lack of an active trading market may make it difficult to
obtain an accurate price for a security. If market conditions make it difficult
to value securities, the Fund may value these securities using more subjective
methods, such as fair value pricing. In such cases, the value determined for a
security could be different than the value realized upon such security's sale.
As a result, an investor could pay more than the market value when buying Fund
shares or receive less than the market value when selling Fund shares. Liquidity
risk may also refer to the risk that the Fund may not be able to pay redemption
proceeds within the allowable time period because of unusual market conditions,
unusually high volume of redemptions, or other reasons. To meet redemption
requests, the Fund may be forced to sell securities at an unfavorable time
and/or under unfavorable conditions.

Management Risk: The investment strategies, practices and risk analysis used by
the Subadvisor may not produce the desired results.

Yield Risk: There can be no guarantee that the Fund will achieve or maintain any
particular level of yield.

Debt Securities Risk: The risks of investing in debt securities include (without
limitation): (i) credit risk, i.e., the issuer may not repay the loan created by
the issuance of that debt security; (ii) maturity risk, i.e., a debt security
with a longer maturity may fluctuate in value more than one with a shorter
maturity; (iii) market risk, i.e., low demand for debt securities may negatively
impact their price; (iv) interest rate risk, i.e., when interest rates go up,
the value of a debt security goes down, and when interest rates go down, the
value of a debt security goes up; (v) selection risk, i.e., the securities
selected by the Subadvisor may underperform the market or other securities
selected by other funds; and (vi) call risk, i.e., during a  period of falling
interest rates, the issuer may redeem a security by repaying it early, which
may reduce the Fund's income, if the proceeds are reinvested at lower interest
rates.

Additional risks associated with an investment in the Fund include the
following: (i) not all U.S. government securities are insured or guaranteed
by the U.S. government-some are backed only by the issuing agency, which must
rely on its own resources to repay the debt; and (ii) the Fund's yield will
fluctuate with changes in short-term interest rates.

High-Yield Securities Risk: Investments in high-yield securities (commonly
referred to as "junk bonds") are sometimes considered speculative because they
present a greater risk of loss than higher quality securities. Such securities
may, under certain circumstances, be less liquid than higher rated securities.
These securities pay investors a premium (a high interest rate or yield) because
of the increased risk of loss. These securities can also be subject to greater
price volatility. In times of unusual or adverse market, economic or political
conditions, these securities may experience higher than normal default rates.

Loan Participation Interest Risk: There may not be a readily available market
for loan participation interests, which in some cases could result in the Fund
disposing of such security at a substantial discount from face value or holding
such security until maturity. In addition, there is also the credit risk of the
underlying corporate borrower as well as the lending institution or other
participant from whom the Fund purchased the loan participation interests.

Floating Rate Loans Risk: The floating rate loans in which the Fund invests are
usually rated below investment grade (commonly referred to as "junk bonds") and
are generally considered speculative because they present a greater risk of
loss, including default, than higher quality debt securities. Moreover, such
securities may, under certain circumstances, be less liquid than higher quality
debt securities. Although certain floating rate loans are collateralized, there
is no guarantee that the value of the collateral will be sufficient to repay the
loan. In times of unusual or adverse market, economic or political conditions,
floating rate loans may experience higher than normal default rates. In the
event of a recession or serious credit event, among other eventualities, the
Fund's investments in floating rate loans are more likely to decline.

Distressed Securities Risk:Investments in distressed securities are subject
to substantial risks in addition to the risks of investing in other types of
high-yield securities. Distressed securities are speculative and involve
substantial risk that principal will not be repaid. Generally, the Fund will
not receive interest payments on such securities and may incur costs to protect
its investment. In addition, the Fund's ability to sell distressed securities
and any securities received in exchange for such securities may be restricted.

Convertible Securities Risk: Convertible securities may be subordinate to other
securities. In part, the total return for a convertible security depends upon
performance of the underlying stock into which it can be converted. Also,
issuers of convertible securities are often not as strong financially as those
issuing securities with higher credit ratings, are more likely to encounter
financial difficulties and typically are more vulnerable to changes in the
economy, such as a recession or a sustained period of rising interest rates,
which could affect their ability to make interest and principal payments. If
an issuer stops making interest and/or principal payments, the Fund could
lose its entire investment.

Equity Securities Risk: Investments in common stocks and other equity securities
are particularly subject to the risk of changing economic, stock market, industry
and company conditions and the risks inherent in the portfolio manager's ability
to anticipate such changes that can adversely affect the value of the Fund's
holdings. Opportunity for greater gain often comes with greater risk of loss.

Foreign Securities Risk: Investments in foreign securities may be riskier than
investments in U.S. securities. Differences between U.S. and foreign regulatory
regimes and securities markets, including less stringent investor protections
and disclosure standards of some foreign markets, less liquid trading markets,
as well as political and economic developments in foreign countries, may affect
the value of the Fund's investments in foreign securities. Foreign securities
may also subject the Fund's investments to changes in currency rates.

Past Performance

Since the Fund does not have a full calendar year of performance as of the date
of this Prospectus, no calendar year performance information is available.

[1] A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within 18 months of the date of purchase on shares that were purchased without an initial sales charge.

[1] "Other Expenses" are based on estimated amounts for the current fiscal year.

[2] New York Life Investment Management LLC ("New York Life Investments") has contractually agreed to waive fees and/or reimburse expenses so that Total Annual Fund Operating Expenses (excluding taxes, interest, litigation, extraordinary expenses, brokerage and other transaction expenses relating to the purchase or sale of portfolio investments, and acquired (underlying) fund fees and expenses) for Class A shares do not exceed 1.05% of its average daily net assets. New York Life Investments will apply an equivalent waiver or reimbursement, in an equal number of basis points, to the other share classes. This agreement will remain in effect until February 28, 2014, and shall renew automatically for one-year terms unless New York Life Investments provides written notice of termination prior to the start of the next term or upon approval of the Board of Trustees of the Fund.

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Dec. 17, 2012
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	MainStay Short Duration High Yield Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks high current income.
Objective, Secondary [Text Block]	rr_ObjectiveSecondaryTextBlock	Capital appreciation is a secondary objective.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds. This amount may vary depending on the MainStay Fund in which you invest. More information about these and other discounts is available from your financial professional and in the "Information on Sales Charges" section starting on page 20 of the Prospectus and in the "Alternative Sales Arrangements" section on page 76 of the Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund's performance. Because the Fund has been in operation for less than one full calendar year, the Fund's portfolio turnover rate for the most recent fiscal year is not available.
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	50,000
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	"Other Expenses" are based on estimated amounts for the current fiscal year.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same.
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund, under normal circumstances, invests at least 80% of its assets (net assets plus any borrowings for investment purposes) in high-yield debt securities that are rated below investment grade by an independent rating agency, such as Standard & Poor's ("S&P"), Moody's Investor  Service, Inc. ("Moody's") or Fitch Ratings ("Fitch"), or that are unrated but are considered to be of comparable quality by MacKay Shields LLC, the Fund's Subadvisor. Debt securities in which the Fund may invest include all types of debt obligations such as bonds, debentures, notes, bank debt, bank loan participations, commercial paper, floating rate loans, U.S. Government securities (including obligations, such as repurchase agreements, secured by such instruments), and convertible corporate bonds. The Fund will generally seek to maintain a weighted average duration of three years or less, although the Fund may invest in instruments of any duration or maturity. Duration is a measure used to determine the sensitivity of a security's price to changes in interest rates. The longer a security's duration, the more sensitive it will be to changes in interest rates. Securities that are rated below investment grade by independent rating agencies are commonly referred to as "junk bonds." These securities are sometimes considered speculative. If independent rating agencies assign different ratings to the same security, the Fund will use the lower rating for purposes of determining the security's credit quality. The Fund may invest up to 20% of its net assets in equity securities, including preferred shares. The Fund also may invest in securities of non-U.S. issuers. The Fund may hold cash or invest in investment grade short-term instruments during times when the Subadvisor is unable to identify attractive high-yield securities. In times of unusual or adverse market, economic or political conditions, the Fund may invest without limit in investment grade securities and may invest in U.S. government securities or other high quality money market instruments. Periods of unusual or adverse market, economic or political conditions may exist in some cases, for up to a year. To the extent the Fund is invested in cash, investment grade debt or other high quality instruments, the yield on these investments tends to be lower than the yield on other investments normally purchased by the Fund. Although investing heavily in these investments may help to preserve the Fund's assets, it may not be consistent with the Fund's primary investment objective and may limit the Fund's ability to achieve a high level of income. Investment Process: The Subadvisor seeks to identify investment opportunities through analyzing individual companies and evaluates each company's competitive position, financial condition, and business prospects. The Fund seeks to minimize interest rate risk through its emphasis on duration management and investments in securities with short and intermediate maturities. The Fund only invests in companies in which the Subadvisor has judged that there is sufficient asset coverage-that is, the Subadvisor's subjective appraisal of a company's value divided by the value of its debt, with the intent of maximizing default-adjusted income and returns. The Subadvisor may sell a security if it no longer believes the security will contribute to meeting the investment objective of the Fund. In considering whether to sell a security, the Subadvisor may evaluate, among other things, meaningful changes in the issuer's financial condition and competitiveness.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Loss of Money Risk: Before considering an investment in the Fund, you should understand that you could lose money. Net Asset Value Risk: The Fund is not a money market fund, does not attempt to maintain a stable net asset value, and is not subject to the rules that govern the quality, maturity, liquidity and other features of securities that money market funds may purchase. Under normal conditions, the Fund's investment may be more susceptible than a money market fund to interest rate risk, valuation risk, credit risk and other risks relevant to the Fund's investments. The Fund's net asset value per share will fluctuate. Market Changes Risk: The value of the Fund's investments may change because of broad changes in the markets in which the Fund invests, which could cause the Fund to underperform other funds with similar objectives. From time to time, markets may experience periods of acute stress that may result in increased volatility. Such market conditions tend to add significantly to the risk of short-term volatility in the net asset value of the Fund's shares. Liquidity and Valuation Risk: Securities purchased by the Fund that are liquid at the time of purchase may subsequently become illiquid due to events relating to the issuer of the securities, market events, economic conditions or investor perceptions. The lack of an active trading market may make it difficult to obtain an accurate price for a security. If market conditions make it difficult to value securities, the Fund may value these securities using more subjective methods, such as fair value pricing. In such cases, the value determined for a security could be different than the value realized upon such security's sale. As a result, an investor could pay more than the market value when buying Fund shares or receive less than the market value when selling Fund shares. Liquidity risk may also refer to the risk that the Fund may not be able to pay redemption proceeds within the allowable time period because of unusual market conditions, unusually high volume of redemptions, or other reasons. To meet redemption requests, the Fund may be forced to sell securities at an unfavorable time and/or under unfavorable conditions. Management Risk: The investment strategies, practices and risk analysis used by the Subadvisor may not produce the desired results. Yield Risk: There can be no guarantee that the Fund will achieve or maintain any particular level of yield. Debt Securities Risk: The risks of investing in debt securities include (without limitation): (i) credit risk, i.e., the issuer may not repay the loan created by the issuance of that debt security; (ii) maturity risk, i.e., a debt security with a longer maturity may fluctuate in value more than one with a shorter maturity; (iii) market risk, i.e., low demand for debt securities may negatively impact their price; (iv) interest rate risk, i.e., when interest rates go up, the value of a debt security goes down, and when interest rates go down, the value of a debt security goes up; (v) selection risk, i.e., the securities selected by the Subadvisor may underperform the market or other securities selected by other funds; and (vi) call risk, i.e., during a  period of falling interest rates, the issuer may redeem a security by repaying it early, which may reduce the Fund's income, if the proceeds are reinvested at lower interest rates. Additional risks associated with an investment in the Fund include the following: (i) not all U.S. government securities are insured or guaranteed by the U.S. government-some are backed only by the issuing agency, which must rely on its own resources to repay the debt; and (ii) the Fund's yield will fluctuate with changes in short-term interest rates. High-Yield Securities Risk: Investments in high-yield securities (commonly referred to as "junk bonds") are sometimes considered speculative because they present a greater risk of loss than higher quality securities. Such securities may, under certain circumstances, be less liquid than higher rated securities. These securities pay investors a premium (a high interest rate or yield) because of the increased risk of loss. These securities can also be subject to greater price volatility. In times of unusual or adverse market, economic or political conditions, these securities may experience higher than normal default rates. Loan Participation Interest Risk: There may not be a readily available market for loan participation interests, which in some cases could result in the Fund disposing of such security at a substantial discount from face value or holding such security until maturity. In addition, there is also the credit risk of the underlying corporate borrower as well as the lending institution or other participant from whom the Fund purchased the loan participation interests. Floating Rate Loans Risk: The floating rate loans in which the Fund invests are usually rated below investment grade (commonly referred to as "junk bonds") and are generally considered speculative because they present a greater risk of loss, including default, than higher quality debt securities. Moreover, such securities may, under certain circumstances, be less liquid than higher quality debt securities. Although certain floating rate loans are collateralized, there is no guarantee that the value of the collateral will be sufficient to repay the loan. In times of unusual or adverse market, economic or political conditions, floating rate loans may experience higher than normal default rates. In the event of a recession or serious credit event, among other eventualities, the Fund's investments in floating rate loans are more likely to decline. Distressed Securities Risk:Investments in distressed securities are subject to substantial risks in addition to the risks of investing in other types of high-yield securities. Distressed securities are speculative and involve substantial risk that principal will not be repaid. Generally, the Fund will not receive interest payments on such securities and may incur costs to protect its investment. In addition, the Fund's ability to sell distressed securities and any securities received in exchange for such securities may be restricted. Convertible Securities Risk: Convertible securities may be subordinate to other securities. In part, the total return for a convertible security depends upon performance of the underlying stock into which it can be converted. Also, issuers of convertible securities are often not as strong financially as those issuing securities with higher credit ratings, are more likely to encounter financial difficulties and typically are more vulnerable to changes in the economy, such as a recession or a sustained period of rising interest rates, which could affect their ability to make interest and principal payments. If an issuer stops making interest and/or principal payments, the Fund could lose its entire investment. Equity Securities Risk: Investments in common stocks and other equity securities are particularly subject to the risk of changing economic, stock market, industry and company conditions and the risks inherent in the portfolio manager's ability to anticipate such changes that can adversely affect the value of the Fund's holdings. Opportunity for greater gain often comes with greater risk of loss. Foreign Securities Risk: Investments in foreign securities may be riskier than investments in U.S. securities. Differences between U.S. and foreign regulatory regimes and securities markets, including less stringent investor protections and disclosure standards of some foreign markets, less liquid trading markets, as well as political and economic developments in foreign countries, may affect the value of the Fund's investments in foreign securities. Foreign securities may also subject the Fund's investments to changes in currency rates.
Risk Lose Money [Text]	rr_RiskLoseMoney	Before considering an investment in the Fund, you should understand that you could lose money.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Past Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	Since the Fund does not have a full calendar year of performance as of the date of this Prospectus, no calendar year performance information is available.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	Since the Fund does not have a full calendar year of performance as of the date of this Prospectus, no calendar year performance information is available.
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Investor Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	3.00%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.65%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.53%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.43%	[3]
Waivers / Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.33%)	[3]
Total Annual Fund Operating Expenses After Waivers / Reimbursements	rr_NetExpensesOverAssets	1.10%	[3]
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2014-02-28
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	409
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	707
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,028
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,934
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	3.00%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.65%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.48%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.38%	[3]
Waivers / Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.33%)	[3]
Total Annual Fund Operating Expenses After Waivers / Reimbursements	rr_NetExpensesOverAssets	1.05%	[3]
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2014-02-28
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	404
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	692
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,002
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,880
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class C
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.65%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other Expenses	rr_OtherExpensesOverAssets	0.53%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.18%	[3]
Waivers / Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.33%)	[3]
Total Annual Fund Operating Expenses After Waivers / Reimbursements	rr_NetExpensesOverAssets	1.85%	[3]
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2014-02-28
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	288
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	650
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,139
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	2,488
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	188
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	650
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	1,139
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	2,488
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class I
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.65%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.48%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.13%	[3]
Waivers / Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.33%)	[3]
Total Annual Fund Operating Expenses After Waivers / Reimbursements	rr_NetExpensesOverAssets	0.80%	[3]
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2014-02-28
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	82
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	326
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	590
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,345
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class R2
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.65%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.58%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.48%	[3]
Waivers / Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.33%)	[3]
Total Annual Fund Operating Expenses After Waivers / Reimbursements	rr_NetExpensesOverAssets	1.15%	[3]
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2014-02-28
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	117
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	436
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	777
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,740

[1] A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within 18 months of the date of purchase on shares that were purchased without an initial sales charge.

[2] "Other Expenses" are based on estimated amounts for the current fiscal year.

[3] New York Life Investment Management LLC ("New York Life Investments") has contractually agreed to waive fees and/or reimburse expenses so that Total Annual Fund Operating Expenses (excluding taxes, interest, litigation, extraordinary expenses, brokerage and other transaction expenses relating to the purchase or sale of portfolio investments, and acquired (underlying) fund fees and expenses) for Class A shares do not exceed 1.05% of its average daily net assets. New York Life Investments will apply an equivalent waiver or reimbursement, in an equal number of basis points, to the other share classes. This agreement will remain in effect until February 28, 2014, and shall renew automatically for one-year terms unless New York Life Investments provides written notice of termination prior to the start of the next term or upon approval of the Board of Trustees of the Fund.

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 14, 2012
Registrant Name	dei_EntityRegistrantName	MAINSTAY FUNDS TRUST
Central Index Key	dei_EntityCentralIndexKey	0001469192
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Dec. 14, 2012
Document Effective Date	dei_DocumentEffectiveDate	Dec. 17, 2012
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Investor Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MDHVX
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MDHAX
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class C
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MDHCX
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class I
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MDHIX
MainStay Short Duration High Yield Fund (Prospectus Summary) | MainStay Short Duration High Yield Fund | Class R2
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MDHRX