Formamdinium lead triiodide (FAPbI₃) perovskite has developed as a promising candidate in solar cells for its excellent optoelectronic property. However, the poor environmental stability is still a critical hurdle for its further commercial application. Element doping is an effective method of improving the stability of FAPbI₃ materials. It has been reported that the FA_1–xCs_xPbI_3–yBr_y stability for heat and water resistance were greatly improved by Cs cations and Br anions co-doping. In this study, we perform first-principles calculations to systematically investigate the crystal structures, electronic structures, and optical properties of FA_1–xCs_xPbI_3–yBr_y. We obtain several stable crystal structures of FA_1–xCs_xPbI_3–yBr_y (x = 0.125, y = 0—0.6) in the cubic phase for different ratios of Cs cations to Br anions. By analyzing the structures of these mixed ion perovskites, it is revealed that the lattice parameters decrease linearly with the increase of concentration of Cs cations and Br anions, which is consistent with previous experimental result. In this work, the formation energy difference (∆E) is calculated and our results show that the mixing of Cs cations and Br anions could increase the thermodynamic stability compared with pure FAPbI₃. The FA_0.875Cs_0.125PbI_2.96Br_0.04 is found to be the most stable in all composites investigated. Furthermore, the band gap, hole and electron effective mass increase with increasing proportion of Br anions, indicating an effective strategy for extending the absorption range of FAPbI₃ perovskites into the ultraviolet of the solar spectrum, thereby affecting the carrier transport mechanism in this material. Density of states (DOS) analysis indicates that the DOS of valence band edge increases with increasing proportion of Br anions and enhancing transitions between the valence and conduction bands. Finally, the absorption rate, carrier collection efficiency, external quantum efficiency, short-circuit current density, open circuit voltage and volt-ampere characteristics for the planar structure perovskite solar cell are analyzed by the equivalent optical admittance method. For the FA_1–xCs_xPbI_3–yBr_y (x = 0.125, y = 0.04, thickness = 0.5—1.0 μm) solar cell, the short-circuit current density and the open circuit voltage are estimated at about 24.7 mA·cm^–2 and 1.06 V. It is demonstrated that the co-doping Cs cations and Br anions can improve the stability of the system without reducing short-circuit current density, which may provide some theoretical guidance in preparing the perovskite solar cells with high efficiency and excellent stability.