First-principles calculations have been performed to study the effect of cation replacement with methylammonium (MA+), Cs+, and Rb+ on the properties of formamidinium lead iodide (FAPbI3) perovskite. It is found that these dopants could improve the stability of the desired α phase of FAPbI3 at reduced temperature by lowering the transition temperature between the perovskite cubic α phase and nonperovskite hexagonal δ phase. Interestingly, the optical absorption properties and the effective masses of holes of FAPbI3 perovskite are only slightly affected. The nature of the improvement of the phase stability resulting from the cation replacement is revealed. However, the calculated mixing energies indicate that these multication materials still suffer long-term instability. Our results provide theoretical guidance for improving current multication engineering strategies or even developing new approaches.