A method for manufacturing methyltetrahydrophthalic anhydride is provided, which includes steps as follows. Maleic anhydride is added into a reactor. Piperylene is added into the reactor, so that the piperylene and the maleic anhydride undergo a first addition reaction. When a conversion rate of the maleic anhydride is more than 25%, the first addition reaction is completed. Isoprene is added into the reactor, so that the isoprene and the maleic anhydride undergo a second addition reaction to obtain a methyltetrahydrophthalic anhydride product. The methyltetrahydrophthalic anhydride product contains 3-methyltetrahydrophthalic anhydride and 4-methyltetrahydrophthalic anhydride.