Quasi-2D CsPbBr3 perovskites hold great potential as the promising emitters for perovskite lightemitting diodes (PeLEDs) owing to their good film morphology, excellent thermal stability and efficient radiative recombination. However, quasi-2D CsPbBr3 films are usually composed of multiple phases with various numbers (n) of [PbBr6] 4 octahedral layers, and the formed n = 1 phase shows nonradiative recombination and retarded exciton transfer, which are detrimental to the performance of PeLEDs. Here, we report a facile approach to grow quasi-2D CsPbBr3/Cs4PbBr6 nanocomposite films as the emissive layers (EMLs) for efficient PeLEDs by simply introducing excess CsBr in the precursor solution. In the perovskite nanocomposite films, it is found that the formation of the n = 1 phase is completely suppressed and the density of trap states is dramatically lowered, leading to the significant reduction of nonradiative losses. In addition, the formed Cs4PbBr6 with a large bandgap can centralize carriers and confine excitons in quantum wells to realize efficient transfer, and can also constrain the crystal size and enhance the exciton binding energy of CsPbBr3, all of which facilitate the radiative recombination. As a result of the reduced nonradiative losses and improved radiative recombination, the green PeLED based on the quasi-2D CsPbBr3/Cs4PbBr6 nanocomposite EML achieves greatly enhanced performance with a maximum external quantum efficiency (EQE) of 11.84%, which is much higher than the maximum EQE of 1.94% of the quasi-2D CsPbBr3 device. This study demonstrates that the construction of perovskite nanocomposites should provide an effective way to promote the performance of PeLEDs.