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In this study, a simple low‐temperature self‐assembly strategy is used to synthesize the CsPbBr3/Cs4PbBr6 nanocomposite with a relatively high photoluminescence quantum yield (PLQY, 72%) and product yield (87%). As expected, the nanocomposite exhibits good photoluminescence (PL) performance and stability. Further, 90% of the PLQY is maintained even when heating at 100 °C for 24 h, and almost no emission degradation is observed after storage in ethanol for 10 days. Furthermore, the prepared CsPbBr3/Cs4PbBr6 nanocomposite exhibits the reversible luminescence phenomenon by simply applying the wetting and drying process. This abnormal phenomenon is due to the internal phase transformation of the CsPbBr3/Cs4PbBr6 nanocomposite. The potential application of the nanocomposite in anti‐counterfeiting has been demonstrated. This work facilitates the production and application of inorganic metal halide perovskite‐based functional materials. [ABSTRACT FROM AUTHOR]