The invention relates to a preparation method and an application of a composite photocatalyst, in particular to a preparation method and an application of a g-C3N4 composite photocatalyst modified with Ni(OH)2 and aims to solve the problems that existing composite photocatalysts are low in catalysis efficiency and the catalysis assisting effects of nickel-based compounds are difficult to directly compare. The method comprises the steps as follows: the g-C3N4 photocatalyst is placed in an NaOH solution to be uniformly mixed; Ni(NO3)2 is dropwise added to the mixed solution; centrifugal collection of a precipitate is performed, dried and placed in a drying oven to be dried, and the g-C3N4 composite photocatalyst modified with Ni(OH)2 is obtained. The photocatalyst is used in comparison of catalysis assisting performance of nickel-based materials in the same catalytic system and photocatalytic decomposition of water into hydrogen.