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-xCs_xPbBr₃ 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.