The Ag3PO4/g-C3N4 composites were prepared by in-situ precipitation method and characterized by XRD, SEM, TEM, UV-Vis-DRS and PL. The results showed that g-C3N4 exhibited a graphite-like and flake-like structure, while Ag3PO4 indicated a spherical-like structure with cubic crystal phase, which were evenly distributed on the surface of g-C3N4. By using methylene blue (MB) as a simulated pollutant, the effect of different molar ratios of g-C3N4/Ag3PO4 on the photdegradation rate were studied. The results showed that when the molar ratio was 1:0.7, the photocatalytic activity was the best, and the MB degradation rate could reach 100% after 30 min of visible light irradiation. The photocatalyst indicated high stability, as the degradation rate retained 85.24% after repeated use of the same photocatalyst for 5 times. The mechanism of degradation research shows that h and e- are the main active substances in photdegrading MB.