the parabolic primary mirror (pm) of space solar telescope is sensitive to the serious external loads and surface variation of pm has large influence on the optical performance of the telescope. to evaluate the pm change under the external loads accurately, the high precision algorithm based on the comprehensive surface parameter is presented. the discrete error of finite element discretization of optical surface is concerned for the first time and the elimination of discrete error is discussed as well. the calculation method of surface parameter including rigid body displacement, surface focus and root mean square is researched. based on the high-precision algorithm, the surface parameter of pm under the isothermal temperature change is calculated and the law curve of surface parameter is also obtained with the change of temperature. the significant simulation result of surface parameter can effectively evaluate the surface change and guide the structure and environmental adaptation design of the pm. (c) 2016 elsevier gmbh. all rights reserved.