Spectral characteristics of a high power laser diode with external fiber grating feedback are investigated both theoretically and experimentally. The theoretical results show that the spectral linewidth can be narrowed or broadened according to different external cavity parameters. The change of spectral linewidth is more sensitive to the effect of external reflection coefficient than the external cavity length. The reduction of spectral linewidth from 6 nm to 0.8 nm is achieved in our experiments. The external cavity with fiber grating feedback not only improves the spectral characteristics but also increases the repression ratio of dominant modes to subordinate modes.