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Zero‐dimensional metal halides have attracted much attention due to their attractive photoelectric properties. Here, we propose a new strategy of synthesizing metal halides crystals by recrystallization in water. The as‐synthesized Cs2InCl5(H2O)‐orange crystals are dissolved and recrystallized in water (Cs2InCl5(H2O)‐blue), with its photoluminescence (PL) changing from orange to blue, both of which are derived from self‐trapping excitons (STEs). The time‐resolved photoluminescence (TRPL) spectrum of Cs2InCl5(H2O)‐blue shows that it has an ultralong lifetime up to milliseconds (τ=52.98 ms), which is expected to be applied in biological sensors. The photoluminescence quantum yield (PLQY) increases from 2.25% to 11.61% in the self‐assembly process. By using a post‐doping method, the PL of crystals turns into red when we introduce Mn2+ as dopant while there is no obvious change upon using a traditional solvent‐thermal method. Recrystallization in water and post‐doping provide a new perspective for the synthesis and doping of metal halides. [ABSTRACT FROM AUTHOR]