The cesium cation (Cs⁺) is widely used as a dopant for highly efficient and stable formamidinium lead tri-halide perovskite (FAPbX₃, X = I, Br, Cl) solar cells. Herein, we introduce a small amount of cesium acetate (CsAc) that can effectively stabilize FAMAPbI₃ under thermal- and light illumination-stress. We show that incorporated Cs⁺ leads to relaxation of strain in the perovskite layer, and that Ac⁻ forms a strong intermediate phase with PbI₂, which can help the intercalation of the PbI₂ film with Cs⁺ and cation halide (FAI, MAI, MACl) in the sequential deposition process. The addition of CsAc reduces the trap density in the resulting perovskite layers and extends their carrier lifetime. The CsAc-modified perovskite solar cells show less hysteresis phenomena and enhanced operational and thermal stability in ambient conditions. Our findings provide insight into how dopants and synthesis precursors play an important role in efficient and stable perovskite solar cells.