All-inorganic cesium lead halide perovskites have shown great potential applications in optoelectronic field due to their fascinating optical properties. Although perovskite materials have achieved great success in various fields, their inherent ionic properties and high dynamic surface properties have led to their poor stability, hindering their applications. The preparation of CsPbBr ₃-Cs ₄PbBr ₆nanocrystals has proven to be an effective strategy to enhance their photoluminescence properties and stability. Herein, we report an easy synthesis of CsPbBr ₃-Cs ₄PbBr ₆nanocrystals with a diphase structure at room temperature by using Cs-OA, Pb-OA and TOABr as precursors in toluene. It is found that the phase transformation and the relative composition between CsPbBr ₃and Cs ₄PbBr ₆are dependent on the concentration of TOABr and the ratio of Cs/Pb. The in-situPL experiments reveal that the formation of ~12 nm CsPbBr ₃nanocubes experiences the fast nucleation, the focusing growth of size-distribution in early growth stage and Ostwald ripening growth in the later stage at a TOABr concentration of 0.16 mmol. With the increase of concentration of TOABr or molar ratio of Cs/Pb > 1 (Cs/Pb < 1), PbBr ₄ ^2–complex and PbBr ₃ ^–complex can coexist and compete with each other in toluene, and the CsPbBr ₃nucleations dominate in the early stage, then CsPbBr _3-Cs ₄PbBr ₆nanocomposites are gradually formed on CsPbBr ₃nucleations as photoluminescence centers due to the continuous generation of PbBr ₄ ^2–complex between TOABr and Pb ²⁺. The relative composition of Cs ₄PbBr ₆in CsPbBr _3-Cs ₄PbBr ₆nanocomposites can be improved from 4% to 85% with the concentration of TOABr increasing or Cs/Pb < 1. The optimized CsPbBr ₃-Cs ₄PbBr ₆composite nanocrystals possess high PLQY and stability. Our work provides an understanding of the mechanism of phase transformation in cesium lead halide perovskite materials.