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Converting and patterning high‐quality perovskite quantum dots (PQDs) into flexible thin films is of great significance for high‐performance solid‐state optical applications. However, the poor stability and low quantum efficiency of PQDs after film formation is a big challenge hindering their usability. Here, an in situ synthesis strategy to prepare ligand‐free long‐term stable CsPb(Br0.3I0.7)3@poly(methylmethacrylate) (PMMA) PQDs fibers with thermal responsive fluorescence performance is demonstrated. The luminescence of the CsPb(Br0.3I0.7)3@PMMA PQDs fibers can rapidly and reversibly quench and recover between heating and cooling cycles. It reveals that the thermally induced phase transition of CsPb(Br0.3I0.7)3 results in this thermally reversible luminescence phenomenon. This temperature‐reversible luminescence characteristic not only deepens the comprehension of the temperature‐dependent phase transition behavior of perovskite materials but also broadens their applications in the fields of information encryption storage, anti‐counterfeiting, temperature warning, and other temperature‐responsive fields. [ABSTRACT FROM AUTHOR]