In order to obtain the optimal design of the φ620 mm ground mirror assembly, the parametric model of mirror and its support system was established and the thickness of mirror, the support position and design parameters of flexible support structure were optimized based on the parametric model. Firstly, the kinematic restraint design principle of the φ620 mm ground primary mirror support system was stated. Then, flexibility matrix and thermal equivalent force of flexible support was deduced from the theoretical perspective, a parametric model of mirror assembly was established to facilitate the optimization iteration, and the parametric model was compared with solid element detailed FEM model in multiple static loadcases in term of the RMS and the first six eigenvalues of the support system. The calculation error was less than 10% of the results to verify the validity of the parameters of the model. Finally, based on the theory of radial basis function approximation model, the multi-objective optimization of the parametric model was possessed in Isight environment. After obtaining the Pareto front of multi-objective optimization, the two objectives of optimal design system was weighed. The proposed optimum design method has better applicability in engineering and can provide reference for the optimization design of the same type of mirror assembly.