Abstract: In this paper, MoO₃ was in-situ grown on g-C₃N₄ (CN) by hydrothermal method, followed by the electrodeposition of Cu₂O to construct a ternary MoO₃-Cu₂O/CN composite photocatalyst. The catalyst was characterized by XRD, SEM, TEM, XPS and FTIR, which proved the successful preparation of the composite photocatalyst. Taking tetracycline as the target pollutant, the degradation effect of the prepared photocatalyst on tetracycline and the action mechanism of the photocatalyst were investigated. The results showed that 1.5 MoO₃-Cu₂O-100/CN composite had the best degradation effect of tetracycline at 150 min under visible light, up to 97.75%. They are 2.2 and 1.5 times of CN (45.28%) and 1.5 MoO₃/CN (63.24%) respectively. The mechanism was investigated by free radical capture experiment and electron paramagnetic resonance spectroscopy (EPR). It was confirmed that hydroxyl radical (·OH) and superoxide radical (·O₂⁻) were the main active substances in the photocatalytic process. The valence band and conduction band positions of CN, MoO₃, and Cu₂O are calculated based on various tests. It is shown that the double Z-type heterojunction is formed by the ternary composite photocatalyst. At the same time, the improvement of photocatalytic activity is mainly due to the construction of double Z mechanism, which widens the visible light absorption range, retains the hole electrons with high REDOX ability, and reduces the recombination rate of photogenerated electrons and holes. The results of stability test show that the catalytic degradation rate of tetracycline is still over 90% after four cycles, which has excellent stability and can be recycled.The results of stability test show that the catalytic degradation rate of tetracycline is still over 90% after four cycles, which has excellent stability and can be recycled.