Abstract: The neutral surface of a concave thin mirror is too close to the mirror surface, which makes it difficult to effectively mount the flexible structure and increases the mirror surface shape error. To address this problem, we design a flexible support structure including connectors, a support plate, and flexible structures, and construct an equivalent mirror by installing connectors and a support plate on the back of the mirror. While ensuring that the neutral surface of the equivalent mirror is moved away from the mirror surface, we optimize the support structure so that the rotary center of the flexible structure is located on the neutral surface of the equivalent mirror, avoiding the tilting moment. Following design and modeling of the structure, we analyze the static and dynamic characteristics using a finite element simulation, finding a root-mean-square (RMS) value for the surface shape error of 9.28 nm under the coupled effects of 1g gravity load, 4°C temperature rise, and 0.005 mm unevenness assembly error, with a fundamental frequency of 170.75 Hz, which all meet the design requirements. Finally, we carry out a surface shape error test of the mirror assembly, confirming it to meet the design index requirement of the mirror assembly. Simulation and test results verify the reliability and effectiveness of our proposed support structure.