The invention belongs to the technical synthesis field of nano materials, and discloses a Ni(OH)2/SrTiO3 composite catalyst and a preparation method and application thereof in photolysis of aquatic hydrogen. SrTiO3 serves as a primary catalyst, Ni(OH)2 serves as a cocatalyst, and the Ni(OH)2 nanoparticle modified SrTiO3 composite catalyst is synthesized through a simple preparation method. The preparation method is easy to operate and control, the product obtained through the preparation method is good in stability, and the photocatalytic effect of SrTiO3 is greatly improved. In an experiment,the molar ratio of Ni(OH)2 in the Ni(OH)2/SrTiO3 composite is 0%, 0.5%, 5%, 10%, 20% and 25% respectively and is labeled as N0, N0.5, N5, N10, N20 and N25 respectively. The result shows that the composite amount of Ni(OH)2 reaches 20% (molar ratio) under UV-vis radiation, the optimal photocatalytic activity for producing H2 in a methanol solution is 1461.7 micromol h(-1) g(-1), which is about 15times that of the hydrogen production rate of SrTiO3 before treatment, and the hydrogen production rate of SrTiO3 before treatment is 97.7 micromol h(-1) g(-1).