A new visible-light-driven heterojunction Ag₃PO₄/g-C₃N₄ was prepared by a simple deposition-precipitation method for the degradation analysis of diclofenac (DCF), a model drug component, under visible-light irradiation. The heterojunction photocatalysts were characterized by a suite of tools. The results revealed that the introduction of Ag₃PO₄ on the surface of g-C₃N₄ greatly promoted its stability and light absorption performance. In addition, the effects of the heterojunction mixing ratios were studied, when the molar ratio of Ag₃PO₄ to g-C₃N₄ in the composite was 30%, the as-prepared photocatalyst Ag₃PO₄/g-C₃N₄ (30%) possessed the best photocatalytic activity toward the photodegradation of DCF, and the optimal photocatalyst showed a DCF degradation rate of 0.453 min⁻¹, which was almost 34.8 and 6.4 times higher than those of pure g-C₃N₄ (0.013 min⁻¹) and Ag₃PO₄ (0.071 min⁻¹) under visible light irradiation (λ ≥ 400 nm). The trapping experimental results showed that h⁺, ·OH, and ·O₂⁻ were the main reactive oxygen species during the photocatalytic reaction. The improved performance of the composites was induced by the high charge separation efficiency of the photogeneration electron-hole pairs as well as the surface plasmon resonance (SPR) endowed in the Ag⁰ nanoparticles, and ultimately enhanced the DCF photodegradation.