Defect states on the zinc oxide (ZnO) surface cause severe interfacial charge recombination and perovskite decomposition during device operation, which inevitably leads to efficiency loss and poor device stability, making the usage of ZnO in perovskite solar cells (PSCs) problematic. Herein, a simple and effective method of inorganic chlorination treatment is used to passivate the surface defects of the ZnO electron transport layer. It is shown that chlorine (Cl) effectively fills the oxygen vacancy defects of ZnO, suppressing charge recombination and facilitating charge transport at the perovskite/ZnO interface. Therefore, the resulting CH3NH3PbI3-based device achieves an enhanced power conversion efficiency with suppressed hysteresis. Meanwhile, the chlorination of the ZnO surface protects the perovskite layer from decomposition, thus improving device stability. Herein, an ingenious method is developed to further improve the device performance of ZnO-based PSCs and useful guidance is provided for the development of other perovskite optoelectronics, especially those with ZnO as the charge transport layer.