The low separation/migration efficiency is a major obstacle that limits the practical application of semiconductor-photocatalysts. Constructing S-scheme heterojunction is an ideal strategy for providing high photocatalytic activity via accelerating charge separation. Herein, an Ag₃PO₄/C₃N₄ composite was synthesized by coupling Ag₃PO₄ particle with C₃N₄ hollow spheres in-situ via a precipitation method. The S-scheme heterojunction between Ag₃PO₄ and C₃N₄ could accelerate the charge separation and retain high photoredox ability, which synchronously realized high photocatalytic oxygen production and hexavalent chromium reduction. The optimized Ag₃PO₄/C₃N₄ composite shows a high oxygen production rate up to 803.31 µmol·g^-1·h^-1 and a high conversion (87.9%) of Cr(VI) to Cr(III). In addition, C₃N₄ hollow spheres affords higher reaction efficiency than that of C₃N₄ tube, C₃N₄ bulk and C₃N₄ sheet, which indicates that the hollow sphere structure can provide more active sites and adsorption sites in the photocatalytic process. This work offers an effective way in developing a dual-function S-scheme heterojunction for clean energy production and environmental protection.