The linewidth characteristics of distributed feedback fiber laser (DFB-FL) is fully explored in this paper. The linewidth broadening effect of DFB-FL induced by self-pulsation, phase-shifted grating, especially lasing window, external disturbance, etc. are extensivly investigated in the paper based on the theoretical derivation and experiment. An effective linewidth compression method is proposed, and a self-injection system of DFB-FL comes up to reduce its linewidth from 35 kHz to less than 10 kHz.