In this work, two photocatalysts (i.e., C₃N₄ and WO₃) were successfully combined into a heterojunction structure by a facile hydrothermal method for mediator-free overall water splitting, analogous to the natural photosynthesis over a two-step photoexcitation Z-scheme system. Hydrogen and oxygen are evolved with a 2 : 1 ratio by irradiating the C₃N₄-WO₃ composites loaded with Pt under visible light (λ > 420 nm) without any redox mediator. Introducing reduced graphene oxide (rGO) into the C₃N₄-WO₃ composites enhances the water splitting efficiency. Through optimizing the mass ratio in the C₃N₄-WO₃ composites, rGO content, amount of loaded Pt and pH value of the reacting system, the highest H₂/O₂ evolution rates of 2.84 and 1.46 μmol h⁻¹ can be obtained, with a quantum yield of 0.9%. Our findings demonstrate that the hydrothermal method is a promising strategy for constructing intimate heterostructures for Z-scheme water-splitting systems without using any redox mediator, and that rGO can be used to further enhance the performance in optimized conditions.