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Perovskite light‐emitting diodes (PeLEDs) based on 3D mixed Br‐Cl compositions have shown good color stability and efficiency in the sky‐blue color range. However, when the Cl/Br ratio increases to reach deep‐blue emission, the performance of the PeLEDs decreases drastically, with the external quantum efficiency (EQE) typically below 1%. Such performance decay is due largely to the poor morphology of the emissive layer. Herein, theoretical and experimental approach is combined to investigate the evolution of solvated lead complexes in the precursor solution of mixed Br–Cl perovskites. It is found that the energetically favorable exchange of halide ions in the lead complexes drives the precipitation of CsCl, leading to extremely low precursor solubility at high Cl/Br ratios and consequently poor perovskite film coverage. Based on the findings, a metastable dissolution strategy for perovskite film preparation is proposed and deep‐blue PeLEDs with a record high EQE of 4% in 3D‐based PeLEDs and excellent spectral stability is achieved.