Quasi-two-dimensional (quasi-2D) perovskites are promising candidates for circularly polarized luminescence (CPL) materials and spin light-emitting diodes (LEDs), owing to their strong spin–orbit coupling (SOC), tunable chiral-inducing structures, and compatibility with device fabrication processes. However, achieving high dissymmetry factors and efficient spin-LED performance remains challenging because single chiral cation systems of quasi-2D perovskites suffer from limited chiral induction and poor phase tunability. In this work, we introduced a mixed-cation strategy by incorporating an achiral organic spacer, 2-phenoxyethanamine (POEA⁺), into quasi-2D perovskites based on a single chiral cation. The incorporation of POEA⁺ not only passivated defects and improved the optical properties of the perovskite films, but also enhanced chirality-induced spin selectivity (CISS) by promoting the formation of intermediate phases and facilitating a more efficient energy transfer process. As a result, the mixed-cation perovskite film exhibited a significantly enhanced CPL signal at 702 nm, with a maximum photoluminescence dissymmetry factor (g_lum) of 4 × 10⁻³ at room temperature. Furthermore, LEDs based on the optimized film demonstrated a high external quantum efficiency (EQE) of 4.22%. This work proposes an effective strategy to simultaneously enhance the chiroptical properties of quasi-2D perovskite and improve the quality of the films, which shows the great potential for future applications in chiroptoelectronics and spintronics.