The surface errors in mounting process of a mirror used in the X-ray focusing telescope with a nested conical approximation Wolter-I type structure was simulated and analyzed for an X-ray Timing and Polarization (XTP) satellite. Firstly, the 2-D finite element model was established based on ANSYS. Then, by taking the actual jig and mounting process as loading and boundary conditions, the curvilinear relationship between surface error and loading on three bars was obtained through the analysis of mounting process of the mirror with different radii and the optimized loadings according to different mirrors were selected. Analysis results show that the minimum surface error is less than 0.1 μm according to the optimized loading and satisfies the accuracy requirement. Moreover, the maximum stresses in the mirror are less than the ultimate strength of glass and maintain the reliability in mounting process. A 3-D model was established to compare with the 2-D model for deviation analysis. Results show that the main calculating deviation is 2.3 μm at front and back sections of the mirror in length about 5 mm. The calculating deviation at middle of mirror is less than 0.03 μm. The research show that the 2-D model could be used in the fast surface error analysis and loading confirmation. According to the analysis of mirror mounting process, the accurate mechanics analysis could provide academic bases and references for improving mounting accuracy.