In this paper, we present an innovative room-temperature ligand-assisted reprecipitation (LARP) strategy, in which the introduction of diethyl ether plays a crucial role in the synthesis of MAPbI3 nanocrystals. Diethyl ether is a weakly polar solvent. Its introduction regulates nucleation and early-stage crystal growth. As a result, enhanced homogeneous nucleation and smaller nanocrystals are obtained. By tuning solvent polarity and optimizing the precursor-to-antisolvent ratio, the addition of diethyl ether not only reduces crystal size and enhances the quantum confinement effect, thereby inducing a pronounced spectral blue shift and enabling emission tunability across the 500–700 nm range, but also suppresses lattice defect formation and passivates surface states, effectively minimizing nonradiative carrier recombination. Consequently, the incorporation of diethyl ether markedly prolongs the average carrier lifetime, improves precursor solution stability, and enhances the overall durability of the nanocrystals, achieving stable light emission. This approach significantly expands the potential of MAPbI3 nanocrystals in optoelectronic applications and provides an effective pathway toward multicolor-tunable and long-term stable light emission.