<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2019-10-07</periodOfReport>

    <noSecuritiesOwned>1</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001169245</issuerCik>
        <issuerName>PhaseBio Pharmaceuticals Inc</issuerName>
        <issuerTradingSymbol>PHAS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001789970</rptOwnerCik>
            <rptOwnerName>Hanson Kristopher</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O PHASEBIO PHARMACEUTICALS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>1 GREAT VALLEY PARKWAY, SUITE 30</rptOwnerStreet2>
            <rptOwnerCity>MALVERN</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19355</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice President, Head of Legal</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <footnotes></footnotes>

    <remarks>Exhibit List - Exhibit 24 - Power of Attorney</remarks>

    <ownerSignature>
        <signatureName>/s/ John Sharp, Attorney-in-fact</signatureName>
        <signatureDate>2019-10-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
