Recently, mixed-cation perovskite has been extensively used for high-performance solar cells. Nevertheless, the mixed-cation perovskite fabricated through the existing method always suffers phase stability and much trap density. Herein, we demonstrate a facile intermediate engineering approach to improve the quality of mixed-cation perovskite. Varying concentrations of MACl is used to treat FAXMA1-XPbI3 intermediates. It is noted that MACl has a strong impact on the crystallization kinetics, charge carrier dynamics as well as defect density of obtained perovskite. The mixed-cation perovskite treated with 20 mg mL-1 MACl yields large grain size, highly uniform morphology, better crystalline stability. Subsequently, the device with acquired high-quality mixed-cation perovskite shows a high efficiency of 20.40%, which is obviously higher than that obtained from the traditional non-treated method. Moreover, the device prepared through the developed method could retain over 85% of the initial efficiency after 860 h at room temperature.