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CsPbBr₃/Cs₄PbBr₆ nanocomposites recently were found to yield efficient luminescence. However, the formation mechanism of the nanocomposites is unclear, and large-scale and green synthesis is still challenging. Here, we develop a self-assembly reaction to fabricate CsPbBr₃/Cs₄PbBr₆ nanocomposites efficiently and environmentally friendly. The transmission electron microscopy clearly shows CsPbBr₃ nanocrystal is embedded in Cs₄PbBr₆ matrix, forming a CsPbBr₃/Cs₄PbBr₆ composite structure. In situ characterization reveals that the CsPbBr₃/Cs₄PbBr₆ nanocomposites are formed by a two-step reaction, driven by ion concentration difference. The self-encapsulation and separation of the CsPbBr₃ NCs by the host Cs₄PbBr₆ result in the material exhibiting a high PLQY of 83% and narrow-band emission at 517 nm with a full width at half-maximum of only 21 nm. Further, we fabricate an on-chip white light …