Ag3PO4 semi-conductor has emerged out as an excellent photocatalyst in removing organic compounds. However, the photo-corrosion and stability of Ag3PO4 limits its application. A series of Ag3PO4/g-C3N4/PVA composite films were prepared by facile ion-exchange and solvent blending method and utilized in degrading gaseous toluene under visible light in this study. Material characterization revealed that Ag3PO4 particles were successfully deposited on the surface of g-C3N4. The results of gaseous toluene degradation showed that 2.6%-Ag3PO4/g-C3N4/PVA composites displayed the superior photo-catalytic performance among the as-prepared samples and the removal efficiency was about 1.35 and 1.26 times higher than that of g-C3N4/PVA and Ag3PO4/PVA, respectively. Additionally, discoloration of x%-Ag3PO4/g-C3N4/PVA composites was detected, and the mechanism of discoloration was discussed. Ag degrees was detected on the surface of composites, indicating that Ag+ of Ag3PO4 was reduced into Ag degrees by reacting with H2O molecules (known as photo-corrosion). The effect of PVA coating on as-prepared samples was significant in leading photo-corrosion as PVA absorbed H2O molecules to the surface of composites. However, PVA coating on the Ag3PO4 was likely to form a chemically crosslinking network by (OH)-O-center dot produced on the surface of Ag3PO4, which further protected Ag3PO4 from photo-corrosion. Humidity in the air played a major role in causing photo-corrosion of Ag3PO4 and hence contributing to the sharp decrease of photocatalytic performance of composites. Moreover, compared to the system with a low humidity (6%), the stability of 2.6%-Ag3PO4/g-C3N4/PVA composite film in degrading gaseous toluene decreased sharply when the humidity was high (70%). This study provided a significant perspective in mechanism of photo-corrosion of Ag3PO4/g-C3N4/PVA composite films and might give some suggestions for further application of Ag3PO4/g-C3N4/PVA in purifying indoor air.