Large-diameter lightweight mirrors with a flexible support structure can reduce the external force loads, inertia loads, heat loads and so on to ensure the quality of the optical imaging system. In order to eliminate the positioning error during polishing, testing and installation process, a novel support technology named flexible displacement support model is used in CCAL polishing. Based on finite element analysis software Ansys, the greatest amount of tilt, the maximum principal stress and the deformation of 1.8 m lightweight reflector caused by CCAL polishing based on the flexible displacement support are analyzed, the simulation and optimization of the diameter and distance of the restriction pad are finished, and the deformation of 1.8 m lightweight reflector in different amounts of springs compression is simulated. Simulation results show that the stiffness of flexible support structure, amounts of tilt of reflector floor plate can meet the polishing condition, maximum principal stress intensity is much smaller than the yield stress of the primary mirror, and the deformation satisfies the processing requirements.