Inorganic–organic hybrid perovskite solar cells (PSCs) have stirred up a new research spree in the field of photovoltaics due to its high photoelectric conversion efficiency and simple preparation process. In recent years, the research of inorganic–organic hybrid PSCs has been widely reported, among which FA⁺/Cs⁺ PSCs are especially outstanding. However, there are few reports explaining the lattice structural change mechanism of Cs_xFA_1−xPbI_1.80Br_1.20 PSCs from the view of chemical bonds. In this work, a facile method of 15% Cs⁺ cations partially substituting FA⁺ cations has been presented to enhance the structural stability and photovoltaic performances of FAPbI_1.80Br_1.20 PSCs. The partial incorporation of Cs⁺ in FAPbI_1.80Br_1.20 resulted in a more beneficial tolerance factor and inhibited the deep defect state of elemental Pb. More importantly, it inhibited the phase transition from the cubic black α-phase to the hexagonal yellow δ-phase of FAPbI_1.80Br_1.20. Moreover, the power conversion efficiency (PCE) of Cs_0.15FA_0.85PbI_1.80Br_1.20 PSCs achieved a substantial improvement. The stability also achieved a remarkable promotion, which was demonstrated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Nuclear Magnetic Resonance (NMR). These analyses indicate that 15% Cs⁺ can induce the lattice shrinkage, reduce the specific traps and inhibit the phase transition, thus improving the structural stabilities of Cs_0.15FA_0.85PbI_1.80Br_1.20 PSCs under atmosphere and calefaction. These results provide an effective way for fabricating stable and efficient inorganic–organic perovskite solar cells with promising properties.