All-inorganic CsPbBr3 perovskite thin films are regarded as promising gain media to realize electrically pumped green laser diodes due to their good stability. However, the amplified spontaneous emission (ASE) of CsPbBr3 perovskite thin films suffers from a relatively high threshold due to their high defect density and large scattering loss. This work demonstrates high-quality CsPbBr3/CsPb2Br5 and CsPbBr3/Cs4PbBr6 thin films using a vapor deposition method. Through rational design and modification on the compositions of thin films, it is found that the existence of CsPb2Br5 or Cs4PbBr6 phase in CsPbBr3 can dramatically improve photoluminescence and lengthen carrier lifetime. The mixed phase films show good crystalline quality with lower trap density and enhanced spatial confinement. As a result, the ASE threshold of CsPbBr3 perovskite thin film is reduced from 12.0 ± 0.2 to 10.1 ± 0.3 µJ cm−2 (with CsPb2Br5) and 10.2 ± 0.2 µJ cm−2 (with Cs4PbBr6). Furthermore, the optical gain is increased from 10.4 ± 0.5 to 21.6 ± 0.9 cm−1(19.5 ± 0.8 cm−1) with the participation of CsPb2Br5 (Cs4PbBr6) phase. The findings provide an effective approach to lower the threshold of CsPbBr3 lasers and shed light on the potential of stable electrically pumped perovskite lasers.