Two-dimensional (2D) semiconductor heterojunctions are considered as an effective strategy to achieve fast separation of photoinduced carriers. Herein, a novel CoWO4/g-C3N4 (CWO/CN) p—n junction was synthesized using an electrostatic self-assembly method. The constructed 2D/2D p—n heterostructure had a rich hetero-interface, increased charge density, and fast separation efficiency of photoinduced carriers. The in-situ Kelvin probe force microscopy confirmed that the separation pathway of photoinduced carriers through the interface obeyed an II-scheme charge transfer mechanism. Experimental results and density functional theory calculations indicated the differences of work function between CWO and CN induced the generation of built-in electric field, ensuring an efficient separation and transfer process of photoinduced carriers. Under the optimized conditions, the CWO/CN heterojunction displayed enhanced photocatalytic H2 generation activity under full spectrum and visible lights irradiation, respectively. Our study provides a novel approach to design 2D/2D hetero-structured photocatalysts based on p—n type semiconductor for photocatalytic H2 generation.