LIMITED POWER OF ATTORNEY

        The undersigned hereby constitutes and appoints F. Ty Edmondson and
Bruce Blefeld, and each of them individually, with full power of substitution,
as the undersigned's true and lawful attorney-in-fact to:

        (1) prepare, execute in the undersigned's name and on the undersigned's
            behalf, and submit to the U.S. Securities and Exchange Commission
            (the "SEC") a Form ID, including amendments thereto, and any other
            documents necessary or appropriate to obtain codes and passwords
            enabling the undersigned to make electronic filings with the SEC of
            reports required by Section 16(a) of the Securities Exchange Act of
            1934, as amended, or any rule or regulation of the SEC;

        (2) execute for and on behalf of the undersigned, in the undersigned's
            capacity as officer and/or director of Zai Lab Limited (the
            "Company"), Forms 3, 4, 5 and any Schedules 13D or 13G in accordance
            with Section 16(a) of the Securities Exchange Act of 1934, as
            amended, and the rules thereunder;

        (3) do and perform any and all acts for and on behalf of the undersigned
            that may be necessary or desirable to complete and execute any such
            Form 3, 4, or 5 or Schedule 13D or 13G, complete and execute any
            amendment or amendments thereto, and timely file such form with the
            SEC and any stock exchange or similar authority; and

        (4) take any other action of any type whatsoever in connection with the
            foregoing that, in the opinion of each such attorney-in-fact, may be
            of benefit to, in the best interest of, or legally required by, the
            undersigned, it being understood that the documents executed by each
            such attorney-in-fact on behalf of the undersigned pursuant to this
            Power of Attorney shall be in such form and shall contain such terms
            and conditions as each such attorney-in-fact may approve in each
            such attorney-in-fact's discretion.

        The undersigned hereby grants to each such attorney-in-fact full power
and authority to do and perform any and every act and thing whatsoever
requisite, necessary, or proper to be done in the exercise of any of the rights
and powers herein granted, as fully to all intents and purposes as the
undersigned might or could do if personally present, with full power of
substitution or revocation, hereby ratifying and confirming all that each such
attorney-in-fact, or each such attorney-in-fact's substitute or substitutes,
shall lawfully do or cause to be done by virtue of this power of attorney and
the rights and powers herein granted.  The undersigned acknowledges that each
foregoing attorney-in-fact, in serving in such capacity at the request of the
undersigned, are not assuming, nor is the Company assuming, any of the
undersigned's responsibilities to comply with Section 16 of the Securities
Exchange Act of 1934, as amended.

        This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, 5, and any Schedules 13D
or 13G with respect to the undersigned's holdings of and transactions in
securities issued by the Company, unless earlier revoked by the undersigned in a
signed writing delivered to each foregoing attorney-in-fact.

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        IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 21st day of December, 2020.

                                        /s/ William Ki Chul Cho
                                        ---------------------------------
                                        Name: William Ki Chul Cho