In order to fully understand the distribution law of thermal stress and gravity during the coating of a large aperture primary mirror, to control the shape change of the reflecting mirror, and improve the quality of film forming, the influence of thermal stress and gravity on the shape of reflecting mirror was theoretically analyzed and studied by finite element analysis method. The results show that the reflecting film of the large aperture primary mirror is more prone to spalling and damage near the edge. During the coating process of a large aperture primary mirror, the shape change of the reflecting mirror caused by its own gravity will have a great influence on the coating quality, and gravity elimination treatment is needed. When the main mirror of 3000 mm diameter is coated by evaporation film material up or down, the support mode needs to be optimized. This study provides a reference for controlling the influence of thermal stress and gravity on the shape of the reflecting mirror when coating a large mirror.