Abstract: The thin catadioptric system included a metal high-order aspherical primary mirror having an 80 mm aperture, and the distance between the optical vertex of primary mirror and the image interface was only 17.5 mm. To assure the layout of all components successfully and meanwhile thicken the mirror body effectively to enhance the stiffness, the primary mirror with 3 mounting ears was designed, and measures of components combination and crisscross mounting frontward in the same circumferential ring were implemented. Moreover, to evaluate the surface figure of metal mirror under mounting stress，bolt pretension was simulated by using the pretension element in ANSYS, and on the basis of modal check, primary mirror models including flexural supporting structure of different thickness were built and wavefront deformation at mounting state was analyzed. Meanwhile, the instance of flatness diversity of mounting interfaces was considered. The schemes of mirror assembly structure with restricted axial distance, the integrated analysis way of mounting stress simulation on wavefront deformation, and measures to decrease the distortion provide the reference on the optomechanical design to increase the mirror surface accuracy after assembling thus assuring the system image quality.