Based on the theoretical knowledge of grating external cavity semiconductor laser, the working principle of Littman-Metcalf type external cavity semiconductor laser was explained, and the wide voltage narrow ground wire and mode selection mechanism of external cavity semiconductor laser were discussed in detail. Based on strict coupling theory and ray transformation matrix, the calculation formula of the influence of system structure parameters on the coupling efficiency of light field was derived. At the same time, several important factors affecting the output linewidth of littman-Metcalf external cavity laser were analyzed, and the variation of linewidth caused by the misalignment of collimating lens in the system was discussed. The calculation results show that the proper control of the parameters of the littman-metcalf grating external cavity semiconductor laser can narrow the intrinsic linewidth of the semiconductor laser with the central wavelength of 785 nm by four orders of magnitude. In this external cavity system, the misalignment of the collimating lens would affect the coupling efficiency between the emitted light field and the feedback light field through the external cavity. Then the output linewidth of the grating external cavity semiconductor laser is affected.