The spontaneous alpha-to-delta phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI(3))(1-x)(MAPbBr(3))(x) to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect alpha-phase perovskite without segregation is x = 0.1-0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 mu s, which is over 20 times of that of FAPbI(3) single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA(+). is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder. Moreover, we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of 6 phase. These findings benefit the understanding of alpha-phase stabilization behavior, and have led to fabrication of perovskite solar cells with highest efficiency of 19.9% via solvent management.