Introducing inorganic cation into hybrid organic-inorganic perovskites (HOIPs) has attracted great attention because of the enhancement stabilities without sacrificing their excellent optoelectronic properties. Here, we introduce Cs and Rb into MAPbI_1.8Br_1.2 single crystals (SCs) to dig out the mixed cation effect on optoelectronic performances and phase stabilities. Both Rb and Cs can increase the lattice capacity, which is sufficient to relieve the lattice stress caused by photon energy, thus achieving the purpose of stabilizing the lattice structure and inhibiting migration of halide ions, compared with MAPbI_1.8Br_1.2 SC. On the other hand, the smaller polarity of Rb and Cs reduces the electron-phonon coupling, thus significantly inhibiting the migration of halide ions. Meanwhile, through planar photo-detectors, MA_0.9Cs_0.1PbI_1.8Br_1.2-based device behaves much excellent optoelectronic performance (R = 0.170 A/W, EQE = 51.39 %, D*= 4.42 × 10¹²Jones, on/off ratio: ~522).