Metal halide perovskites have received considerable attention in the field of electroluminescence,and the external quantum efficiency of perovskite lightemitting diodes has exceeded 20％.CH3NH3PbBr3 has been intensely investigated as an emitting layer in perovskite light-emitting diodes.However,perovskite films comprising CH3NH3PbBr3 often exhibit low surface coverage and poor crystallinity,leading to high current leakage,severe nonradiative recombination,and limited device performance.Herein,we demonstrate a rationale for composition engineering to obtain high-quality perovskite films.We first reduce pinholes by adding excess CH3NH3Br to the actual CH3NH3PbBr3 films,and we then add CsBr to improve the crystalline quality and to passivate nonradiative defects.As a result,the (CH3NH3)1-xCsxPbBr3 based perovskite light-emitting diodes exhibit significantly improved external quantum and power efficiencies of 6.97％ and 25.18 lm/W,respectively,representing an improvement in performance dozens of times greater than that of pristine CH3NH3PbBr3-based perovskite light-emitting diodes.Our study demonstrates that composition engineering is an effective strategy for enhancing the device performance of perovskite light-emitting diodes.