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Formamidinium lead iodide (FAPbI3) is ideal for highly efficient and operationally stable perovskite solar cells (PSC). However, a primary challenge for FAPbI3 PSC is to suppress the phase transition from the photoactive black phase into the yellow nonperovskite δ-phase. The preparation of Cs-containing mixed FAPbI3 perovskite by cation stoichiometric engineering is demonstrated and the influence of the Cs/FA ratio on its phase stability and device performance is discussed. By exploring the optimal ratio of Cs and FA cations in Cs x FA1−x Pb(I0.94Br0.06)3 perovskite, an inverted planar device with Cs0.17FA0.83Pb(I0.94Br0.06)3 composition shows the best power conversion efficiency (PCE) of 16.5% in an active area of 1.08 cm2. More importantly, the Cs0.17FA0.83Pb(I0.94Br0.06)3 perovskite photoactive layer showed remarkable long-term stability, maintaining 88.1% of its initial efficiency for 1128 h in the presence of moisture and oxygen and without any encapsulation. The excellent long-term stability is found to originate from the appropriate tolerance factor and low thermodynamic decomposition energy, which underpins the strong potential for the commercialization of Cs-containing mixed FAPbI3 PSCs.