In this paper, ZnO/g-C3N4 series composite photocatalysts were prepared by thermal polymerization and hydrothermal method. The structure, morphology and light absorption properties of the prepared samples were characterized and tested. The photocatalytic performance of these samples was evaluated by the photocatalytic degradation of methylene blue (MB) and the photolysis aquatic hydrogen under visible light irradiation. The experimental results indicate that the performances of the ZnO/g-C3N4 composite photocatalysts are superior to those of ZnO and g-C3N4. Moreover, when the molar ratio of ZnO and g-C3N4 is 1∶1, the photocatalytic performance of the prepared ZnO/g-C3N4 composite samples is the best. On the one hand, the degradation rate of the 1∶1 composite sample reaches 94.36% after only 30 min, and the degradation rate is 5.6 and 6.7 times that of ZnO and g-C3N4, respectively. On the other hand, the photolysis aquatic hydrogen of the 1∶1 sample after 6 h is 11.75 mmol, and the photolysis aquatic hydrogen rate is 7 times that of g-C3N4. The results show that the ZnO/g-C3N4 composite photocatalyst has excellent performance in pollutant degradation and photolysis aquatic hydrogen. In addition, the photocatalytic degradation and photolysis aquatic hydrogen mechanism of ZnO/g-C3N4 series composite photocatalyst are also studied.