According to the working principle of the telescope, we know that the telescope requires stray light from the system to reach the order of 10−10 of the output laser power. In this article, given the roughness of the M1 mirror of 3 Å and the roughness of the M2∼M4 mirror of 1.8 Å, through separate analysis of the four mirror surfaces, we found that M4 has the greatest impact on the backward stray light of the telescope, and as the angle of M4 incident light increases, the level of stray light in the system decreases; after adjusting the M4 incidence angle and considering only the roughness, the stray light level of the telescope system reaches 10−11 of the power of the outgoing laser, which meets the expected requirements. Subsequently, we calculated the impact of particle pollution on the stray light of the system, and based on our analysis results, we determined that the cleanliness level of the telescope testing and storage environment was better than 100. Then, we conducted surface defect calculations and obtained the surface defect requirements for M1 to M4, and it is concluded that as the scattering angle decreases, the main contribution of bidirectional reflectance distribution function (BRDF) changes from geometric optics to diffraction effects. Finally, we conducted actual measurements on the surface quality of the ultra-smooth mirror sample, and the measured BRDF value was substituted into the simulation analysis, resulting in a telescope stray light of 8.29×10−11, meeting the expected requirements.