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Metal halide perovskites exhibit small exciton binding energy, which leads to a low electron–hole capture rate for radiative recombination and accordingly decreases the luminescence efficiency. Reducing the thickness of the perovskite film or the size of the perovskite crystal is found to be an effective method to spatially confine the electrons and holes to promote the bimolecular radiative recombination. Here, we fabricate CsPbBr3/Cs4PbBr6 nanocomposites, applicable for light emission diodes, by a simple self-assembly method. We effectively reduce the critical size of the CsPbBr3 nanocrystals in the CsPbBr3/Cs4PbBr6 nanocomposites by adding a certain amount of dimethyl sulfoxide into the perovskite precursor solution. Accordingly, the photoluminescence quantum yield of the CsPbBr3/Cs4PbBr6 nanocomposites increased from 56 to 91% due to the quantum size effect. In situ observation of the growth of CsPbBr3/Cs4PbBr6 nanocomposites reveals that the reduction of the CsPbBr3 crystal size is due to the change of the chemical reaction speed during the two-step growth process of the CsPbBr3/Cs4PbBr6 nanocomposites.