InP-based quantum dots (QDs) are widely adopted as a superior alternative to CdSe-based QDs in various fields owing to their high quantum yield, environmental friendliness, and excellent stability. However, improving its color purity remains a challenging task. In this work, we employ a multistage heating strategy to optimize the nucleation and shell growth processes of amino-phosphine-based InP/ZnSe/ZnS QDs for reducing emission linewidths. The multistage heating strategy mitigates the undesired formation of small-size cores by decreasing monomer supersaturation during the nucleation process, thereby promoting size-focusing growth. During the shelling process, multistage heating effectively suppresses Zn2+ diffusion into the InP core while ensuring high-quality shell growth, thus reducing the homogeneous broadening caused by exciton-phonon coupling. Compared to classical synthesis, the multistage heating strategy can reduce the emission linewidth of nucleation and shelling by 13.2% and 30.9% respectively. The optimized InP/ZnSe/ZnS QDs exhibit a narrow full width at half maximum (FWHM) of 41.5 nm at 630 nm, representing significant progress in studying spectral linewidths of amino-phosphine InP QDs. This work provides potential insights for further improving the spectral linewidth performance of InP QDs or other nanocrystals with similar reaction-limited growth systems.