This study tackles the significant challenge of phase separation in mixed halide (Br^–/Cl^–) perovskite systems, which severely affects the spectral stability of blue perovskite light-emitting diodes (PeLEDs). A compositional engineering strategy is proposed, precisely controlling the Cs:Pb molar ratio (1∶1 to 1.1∶1) in precursor solutions to construct a CsPb(Br_1–xCl_x)₃/Cs₄Pb(Br_1–xCl_x)₆ composite phase structure. Transmission electron microscopy (TEM) mapping and X-ray diffraction (XRD) analysis confirm that Cs₄Pb(Br_1–xCl_x)₆ nanocrystals (5–8 nm in diameter) grow in situ and uniformly encapsulate CsPb(Br_1–xCl_x)₃ microparticles (50–100 nm). This composite architecture has double functional advantages: 1) the Cs₄PbX₆ shell acts as a physical barrier, reducing halide ion migration activation energy and suppressing phase segregation during continuous operation; 2) the wide-bandgap (3.9–4.3 eV) Cs₄PbX₆ induces quantum confinement effects, confining carriers within CsPbX₃ while passivating defect states, thereby improving perovskite performance. The optimized PeLED achieves notable improvements in brightness, external quantum efficiency, and operational stability, maintaining stable emission at 478 nm under a 50 mA/cm² current density. This is achieved by inhibiting halide phase separation and enhancing the efficiency of carrier recombination achieved by the cesium-lead halide heterojunction system. This work provides fundamental insights into phase-stable perovskite design via composite crystallization kinetics, providing a viable pathway toward commercial-grade blue PeLEDs for full-color displays.