Spectral linewidth narrowing of gain-switched distributed feedback laser diode with pulse injection seeding is related to injection time, as indicated by experiment and theoretical analysis. The experimental and numerical simulation results show that in an approximately 100 ps time-window before the generation of optical pulse, the spectral linewidth can be narrowed dramatically by injecting pulse seeding, and that spectral linewidth decreases with increasing intensity of injection pulse. Near transform-limit low-chirp ultrashort optical pulses can be obtained with optical pulse injection seeding. A spectral linewidth reduction from 0.46 to 0.08 nm was obtained and the time-bandwidth product of 2.46 lowered down to 0.70 correspondingly in the experiment.