The effect of polyethyleneglycol-200 (PEG-200) post-treatment on common commercial PEDOT:PSS buffer layer for perovskite solar cell has been carefully investigated. Through facile annealing post-treatment of PEG/PEDOT:PSS film at 120 degrees C for 10 min, the conversion efficiency of the cells can be enhanced 7.5% (11.68% to 12.56%) compared to the traditional inverted perovskite solar cell, with the improvement of both short-circuit current density and fill factor. Our results indicate that the treatment by PEG on PEDOT:PSS film is an effective way to improve perovskite solar cell efficiency. In addition, it reveals that the PEG-200 can greatly enhance not only the conductivity but also improve the buffer layer morphology or PEDOT:PSS polymer chain packing. Furthermore, the devices impedance spectroscopy reveals that the PEG post treatment would decrease R ct drastically, suggesting the improvement of contact interface between PEDOT:PSS and perovskite layer.