Based on the working principle of Littman-Metcalf type grating external cavity semiconductor laser, an external cavity structure which can reduce diffraction loss is designed. Based on the Littman-Metcalf structure, a reflector is added to feed back the zero-order diffraction light generated by the secondary diffraction of the shining grating to the intrinsic cavity of the semiconductor laser. The expression of the external cavity loss of the new structure model is derived, and the external cavity loss, threshold current, output line width and output power of the two laser structures are simulated by the concept of equivalent cavity. The results show that the zero-order light fed back to the active region can effectively reduce the external cavity loss of the Littman-Metcalf structure laser and improve the coupling efficiency of the system, thus reducing the threshold current and improving the output power of the laser. At the same time, the output linewidth of the laser is further narrowed by improving the reflection efficiency of the external cavity. The factors affecting the output linewidth and output power of low loss Littman-Metcalf external cavity laser (end reflectivity, internal and external cavity length, blazed grating diffraction efficiency and mirror reflectivity) are also simulated and analyzed. It improves the theoretical guidance for the later laser production.