Ag2O/PEDOT:PSS used as the anode buffer layer is introduced into the P3HT:PCBM based polymer solar cell (PSC). Effect of the Ag2O/PEDOT:PSS composite anode buffer layer on the device performance is investigated. According to the results, we can find that the post-thermal annealing can improve the performance of the PSC with Ag2O/PEDOT:PSS anode buffer layer compare with that without buffer layer. In addition, compared with the devices without such a buffer layer or with only PEDOT:PSS buffer layer, the device with the composite buffer layer can achieve higher Jsc, external quantum efficiency as well as power conversion efficiency. We conclude that the post-thermal annealing can significantly improve the surface morphology which increases the light absorption and the exciton dissociation. The inserted Ag2O together with PEDOT:PSS as the composite buffer layer not only efficiently lowers the hole extraction barrier and improves the hole collection efficiency but also exhibits excellent stability.