Metal halide perovskites, particularly quasi-2D perovskites, have emerged as promising candidates for next-generation laser diode gain media due to their exceptional optoelectronic properties. However, conventional quasi-2D perovskites suffer from inefficient exciton funneling and pronounced efficiency roll-off at high carrier densities. Here, a quasi-2D/3D perovskite structure is proposed with a high-efficient energy cascade, modulated through molecular engineering strategy. The C─O─C functional groups in PEO form hydrogen bonds with PEA⁺, thereby delaying the assembly of PEA⁺ with the [PbBr₆]⁴⁻ octahedra inorganic layer. This modification led to refined grain size, enhanced crystallinity, and improved surface flatness in the resulting films. Furthermore, the engineered quasi-2D/3D thin film exhibits an increased exciton binding energy while alleviating efficiency roll-off at high carrier density, achieved by effectively suppressing Auger recombination through directional energy transfer from the quasi-2D to the 3D phase. Consequently, the amplified spontaneous emission threshold of quasi-2D/3D films is reduced to 16.6 µJ cm⁻², and obtained a higher net modal gain coefficient (892 cm⁻¹). These findings provide critical insights for developing low-threshold perovskite lasers.