<p>Solution-processable quasi-2D perovskites are regarded as promising candidates for laser gain media due to their superior exciton binding energy and stability compared to their 3D counterparts. However, quasi-2D perovskites produced through conventional methods often display a distribution of <i>n</i>-value phases, arising from the necessity of multiple precursors to react. The presence of impurity phases may introduce interfacial defects and energetic disorder, which can hinder charge injection into the desired emission centers. In this study, we present the first demonstration of stimulated emission from a phase-pure quasi-2D perovskite achieved through a solvent-sieving method for selective phase removal. This approach enables the dynamic purification of quasi-2D perovskites (<i>n</i> = 8) and significantly lowers the threshold for amplified spontaneous emission. X-ray diffraction and ultraviolet-visible characterization reveal a nearunity <i>n</i> = 8 phase (99.85%) with no detectable low-<i>n</i> phases in our samples. Additionally, the phase-pure quasi-2D perovskite film exhibits a narrower and more intense 001 peak (full width at half maximum (FWHM) of 0.16 nm, intensity of 16,129) compared to that of the pristine quasi-2D perovskite film (FWHM of 0.22 nm, intensity of 3304), indicating an increased grain size and enhanced crystallinity. This improvement leads to a prolonged charge carrier lifetime of approximately 6.98 ns, suggesting a reduced trap density. Furthermore, the phase-pure perovskite film is nearly pinhole-free and features an exceptionally flat surface with a root mean square roughness of 1.18 nm. Consequently, the phase-pure quasi-2D perovskite demonstrates a lower amplified spontaneous emission threshold of approximately 13.82 µJ cm⁻², representing a 12.5% reduction compared to conventional mixed-phase films. This work offers an efficient method for producing pure-phase quasi-2D perovskite suitable for low-threshold lasers.</p>