The invention discloses a preparation method of a graphene-supported Pd nanoflower catalyst and a product of the graphene-supported Pd nanoflower catalyst and an application in a formic acid fuel cell. A Pd nanoflower which is uniform in size and distribution directly grows on the graphene surface through modifying a phosphomolybdic acid on the graphene surface by a method of reducing H2PdCl4 through an ascorbic acid. According to the method, no surfactant is added; the overall synthetic process is carried out in a water solution at a room temperature and atmospheric pressure; and any complicated instrument is not needed. The Pd nanoflower of the prepared catalyst can be firmly combined on the graphene surface; and the dispersity is relatively high. The catalyst is applied to the formic acid fuel cell, has very excellent formic acid oxidation catalysis activity and outstanding stability, can be used as a positive catalyst of a direct formic acid fuel cell, and has a very broad application prospect.