In this work, cobalt phosphide (CoP) nanoparticles were successfully decorated on an ultrathin g-C3N4 nanosheet photocatalysts by in situ chemical deposition. The built-in electric field formed by heterojunction interface of the CoP/g-C3N4 composite semiconductor can accelerate the transmission and separation of photogenerated charge-hole pairsand effectively improve the photocatalytic performance. TEM, HRTEM, XPS, and SPV analysis showed that CoP/g-C3N4 formed a stable heterogeneous interface and effectively enhanced photogenerated electron-hole separation. UV-vis DRS analysis showed that the composite had enhanced visible light absorption than pure g-C3N4 and was a visible light driven photocatalyst. In this process, NaH2PO2 and CoCl2 are used as the source of P and Co, and typical preparation of CoP can be completed within 3hours. Under visible light irradiation, the optimal H-2 evolution rate of 3.0mol% CoP/g-C3N4 is about 15.1 mu molh(-1). The photocatalytic activity and stability of the CoP/g-C3N4 materials were evaluated by photocatalytic decomposition of water. The intrinsic relationship between the microstructure of the composite catalyst and the photocatalytic performance was analyzed to reveal the photocatalytic reaction mechanism.