The realization of colloidal alloy quantum dots (QDs) with narrow spectral linewidths requires minimization of the contributions of inhomogeneous and homogeneous broadening to the ensemble spectrum. Recently, there has been remarkable progress in eliminating the inhomogeneous contribution by controlling the size distribution of the QDs. However, considerable challenges remain in suppressing the homogeneous broadening, in terms of both intrinsic principles and rational synthetic routes. We find that ground-state exciton fine structure splitting and exciton–phonon coupling play a pivotal role in the homogeneous broadening mechanism. Here we demonstrate that the elimination of the lattice mismatch strain by using a coherent strain structure can decrease the light-heavy hole splitting, thus suppressing the asymmetric broadening of the emission on the high energy side. Besides, the improvement of the uniformity of the alloy by using a stepwise ion exchange strategy can weaken the exciton–longitudinal optical (LO)-phonon interactions, further minimizing the homogeneous broadening. As a result, the final alloy QD products exhibit a widely tunable blue emission wavelength (445–470 nm) with the narrowest ensemble photoluminescence full width at half maximum (FWHM) of 10.1–13.5 nm (or 58.4–75.3 meV). Our study provides a potential strategy for other semiconductor nanocrystals with ultranarrow spectral linewidths.