A semiconductor photocatalyst serves as the primary component in photocatalytic oxidation. Scientists are known to be working on modifying existing catalysts and developing new efficient photocatalysts. In recent years, g-C₃N₄ has attracted wide attention due to its high stability, non-toxicity, cost-effectiveness, and adjustable band-gap energy. However, g-C₃N₄ also has shortcomings such as ineffcient visible light absorption, a high recombination rate of photoinduced electrons and holes and low quantum efficiency, which significantly restrict its photocatalytic activity. Here, an innovative ternary composite photocatalyst CdS@Cu/g-C₃N₄ has been successfully fabricated using a simple method, and the photocatalytic degradation of methylene blue (MB) by the CdS@Cu/g-C₃N₄ composite photocatalyst was also studied. Systematic studies showed that the photocatalytic degradation rate of CdS@Cu/g-C₃N₄ on MB reached 85.19% within 20 min, which was 2.58 and 1.88 times higher than that of CdS and g-C₃N₄, respectively. Free radical capture experiments showed that ·O₂⁻ plays a significant role during the photocatalytic process. It is postulated that a type II heterojunction might be formed between CdS and g-C₃N₄, effectively restricting photoinduced carrier recombination and enhancing visible light absorption. Cu doping changes the optical properties, affects the energy band structure of g-C₃N₄, increases the efficiency of electron transfer and improves the electron/hole separation rate, which helps to improve the photocatalytic activity. This work provides a valuable strategy to improve the photocatalytic performance of g-C₃N₄.