The key step of photocatalytic technology using light energy to split water for hydrogen production is to prepare high performance and environmentally friendly stable photocatalysts. In this study, Co₃O₄ was loaded onto polymeric carbon nitride (PCN) to construct S-scheme heterojunctions by an impregnating method. The successful construction of S-scheme Co₃O₄/PCN heterojunctions was proved by the various characterizations, the constructed S-scheme Co₃O₄/PCN heterojunctions have favorable photoelectric effect and exhibit better H₂ evolution performance than the single Co₃O₄ and PCN. Construction of S-scheme Co₃O₄/PCN heterojunctions can elevate active sites induced by N-deficient formation. Therein, 3% is the optimal molar ratio of Co/PCN, and the corresponding sample (3CoO/CN) exhibits the highest H₂ evolution efficiency (105.06 μmol·g ^− 1·h ^− 1) via simulated solar light illumination for 4 h, which is about 41 folds higher than the single PCN (2.48 μmol·g ^− 1·h ^− 1). In short, the conspicuously boosted water splitting efficiency is mainly due to the strong light responsiveness and rapidly separation and migration of photoexcited charge pairs. The cyclic experiment results show that the S-scheme Co₃O₄/PCN heterojunction photocatalysts have high stability. The apparent quantum efficiency (AQE) of H₂ generation on 3CoO/CN is 0.053% under 420 nm monochromatic light irradiation. This study strongly verifies that construction of S-scheme Co₃O₄/PCN heterojunctions is a feasible route to boost the photocatalytic activity of g-C₃N₄, providing an interesting reference for design highly efficient and environmentally friendly C₃N₄-based photocatalysts.