Abstract: The off-axis three-mirror optical system has high requirements for the surface shape accuracy of its optical off-axis mirror assemblies. A flexible assembly structure was designed for the mirror assembly of a common-path off-axis three-mirror optical system to reduce the influence of screw preloading on the surface shape accuracy of the mirror during assembly. Firstly, a solid finite element model of the screw assembly of the mirror was established, and the displacement response of the mirror under the screw preloading condition was calculated through simulation. Subsequently, the surface shape deviation of the mirror was calculated based on the deformation displacement obtained after removing the rigid body displacement. Finally, the effect of the proposed stress-relief structure on ensuring the assembly accuracy of the off-axis three-mirror optical system wass verified by comparing with the traditional rigid assembly structure without stress-relief design. For the two experimental comparative toolings with and without stress-relief design, under the condition of applying the same pre-tightening torque of 4 N·m to the screws, interferometric measurements revealed significant improvements in surface accuracy for the stress-relief design: reducing RMS values of the primary mirror from 0.051 λ to 0.026 λ (reduction of 49.02%). Moreover, the experimentally measured RMS values are all between the simulation results, verifying the effectiveness of the simulation results and that the designed stress-relief structure can effectively reduce the influence of screw preloading on the surface shape of optical components during assembly, thereby ensuring that the off-axis three-mirror optical system meets the design requirement for high surface shape accuracy.