Although pure formamidinium iodide perovskite (FAPbI) possesses an optimal gap for photovoltaics, their poor phase stability limits the long-term operational stability of the devices. A promising approach to enhance their phase stability is to incorporate cesium into FAPbI. However, state-of-the-art formamidinium-cesium (FA-Cs) iodide perovskites demonstrate much worse efficiency compared with FAPbI, limited by the different crystallization dynamics of formamidinium and cesium, which result in poor composition homogeneity and high trap densities. We develop a novel strategy of crystallization decoupling processes of formamidinium and cesium via a sequential cesium incorporation approach. As such, we obtain highly reproducible, highly efficient and stable solar cells based on FA Cs PbI ( = 0.05-0.16) films with uniform composition distribution in the nanoscale and low defect densities. We also revealed a new stabilization mechanism for Cs doping to stabilize FAPbI, i.e. the incorporation of Cs into FAPbI significantly reduces the electron-phonon coupling strength to suppress ionic migration, thereby improving the stability of FA-Cs-based devices.