Inverted polymer solar cells with P3HT:PCBM as active layer are fabricated based on poly(dopamine)/ZnO (PDA/ZnO) as composite cathode buffer layer. Effects of PDA/ZnO composite cathode buffer layer with the different self-polymerization times on the device performance are investigated. According to the results, the short circuit current and photoelectric conversion efficiency of polymer solar cells first increase then decrease with the increase of the self-polymerization time of PDA. For 10-min PDA self-polymerization, the photovoltaic performance of the device achieves the optimal values:open circuit voltage 0.66 V, short circuit curent density 9.70 mA/cm2, fill factor 68.06%, and power conversion efficiency 4.35% under irratiation of light with a strength of 100 mW/cm2. We conclude that the improvement of device performance is due to the PDA/ZnO composite cathode buffer layer reduced the contact resistance between the ZnO and ITO, at the same time, the presence of a large number of nitrogen groups in PDA is advantageous for the electronic collection of the inverted polymer solar cells. Meanwhile, polymer solar cell with PDA/ZnO as composite cathode buffer layer also exhibits excelent stability. In addition, PDA has a strong adhesive force that makes the ZnO interface layer on its surface not easy to fall off. This provides a new way of fabricating the flexible polymer solar cell devices.