The theory of grating external cavity semiconductor lasers is discussed, and the influences of different factors on the output power and linewidth reduction of the Littrow-type external cavity semiconductor lasers are numerically simulated. The Littrow-type grating external cavity semiconductor laser is set up, and single longitudinal mode and highly-equality laser output is realized. The continuous output power is achieved with 180 mW at working current of 400 mA and the spectrum linewidth is less than 1 MHz.