Abstract: Surface shape accuracy under gravity is an important aspect of space-based mirror performance. A 3 m aperture space-based mirror was optimized based on the parametric model of mirror structure and support point distribution, taking the surface shape RMS under the action of gravity as the object function. Firstly, the number and location of the support points were determined by using the classical theoretical formula, and the structure of the mirror was designed preliminarily. Secondly, according to the lightweight characteristics of the mirror and the kinematics design of the support system, the parameterized model of the mirror structure and the support points' distribution was established. Finally, the process integration and process automation of mirror component optimization were realized by using Isight software, and the relationship between the shape accuracy under gravity and various parameters was studied. The results show that the RMS value of the optimized mirror is 86.7 nm, which is 66.6% less than that of the preliminary design 259.4 nm, and meets the requirements of the project. The proposed optimization method combines with the kinematic model of the support system, and provides a comprehensive and efficient new approach for the optimization of large aperture mirrors with similar lightweight structure and support schemes.