In summary, we successfully develop a novel sequential Cs incorporation strategy to tackle the critical challenge on different crystallization dynamics of different cations in developing FA1-xCsxPbI3 perovskite PSCs. The resulting pure iodide FA1-xCsxPbI3 perovskites show uniform composition distribution and reduced defects/traps density. Compared with FAPbI3, the FA0.91Cs0.09PbI3 exhibits reduced electron-phonon coupling and lattice fluctuations, minimizing ion migration and hence enhancing the stability. As such, we have been able to achieve highly stable PSCs with a high efficiency of 24.7%, a record for FA1-xCsxPbI3 PSCs. This work opens up new possibilities to develop high-quality mixed cation perovskites, presenting a milestone towards the development of highly efficient and highly stable perovskites for various applications, including solar cells, light-emitting diodes and lasers.