A narrow band diode laser with novel structure of external cavity is reported in this paper. With weak optical feedback, the laser linewidth was narrowed to below 500 kHz. This diode laser can be used for the study of high resolution spectroscopy, magnetic optical trap and atomic fountain frequency standards etc. In addition, we studied theoretically and experimentally the relation between linewidth and current tuning rate in detail. The obtained formula was proved by means of saturated spectroscopy.