Perovskite light-emitting diodes (PeLEDs) have achieved remarkable progress in recent years, with green-emitting PeLEDs now exhibiting external quantum efficiencies (EQEs) exceeding 30%, rivaling those of organic light-emitting diodes (OLEDs). While quasi-2D perovskite structures have emerged as a promising strategy for high-efficiency PeLEDs─owing to their enhanced exciton binding energies and charge carrier confinement─their phase distribution remains a critical yet challenging factor governing device performance. In this study, we demonstrate that the phase distribution in quasi-2D perovskite films is highly sensitive to the solvent atmosphere during annealing. Polar solvents (e.g., dimethylformamide, dimethyl sulfoxide) introduce nonradiative recombination centers and accelerate film decomposition, whereas nonpolar solvents (e.g., chlorobenzene) suppress degradation and promote high--phase purity. By employing nonpolar solvent vapor post-treatment, we achieve enhanced radiative recombination efficiency, yielding PeLEDs with a maximum EQE of 24.27%. Our findings highlight a previously overlooked aspect of perovskite film fabrication and provide key insights for the scalable production of quasi-2D PeLEDs.