The invention belongs to the technical field of nano material synthesis. A graphite-like phase nitrogen carbide (g-C3N4) nanosheet photocatalyst is synthetized in one step by using a simple, direct and quick thermal polymerization method, and can be used for photocatalytic decomposition on water to produce hydrogen under visible light. The method comprises the following steps: weighing tripolycyanamide, putting into a round crucible, flatly putting treated foam nickel on the tripolycyanamide, covering with a crucible cover, flatly putting into a muffle furnace with initial temperature of 50 DEG C, heating the muffle furnace to 500 to 550 DEG C at a heating rate of 2.3 DEG C/min, reacting for 4h at the temperature which is 550 DEG C preferably, and naturally cooling to room temperature, thus obtaining black foam nickle on an upper layer and a corresponding nitrogen carbide nanosheet on a lower layer.