The semi-active support is based on the semi-active optical technology, and the correction force is converted into a correction torque through a Warping Harness(WH) spring blade to correct the mirror low-order aberration introduced by error sources such as gravity and temperature. Aiming at the defects of traditional empirical design of mirrors, a new optimal design method for a mirror support system is proposed, that is, a comprehensive design optimization method of mirror support system based on structural size optimization combined with empirical design, and a set of semi-active mirror support systems based on WH is established. Firstly, the initial structure of the support system is designed according to the empirical formula; an L-shaped hollow WH spring blade is designed, and the nonlinear analysis and fatigue analysis are carried out to determine that the blade thickness is 2 mm and the service life is 1.2×10⁶ times. Then, the RMS value of the mirror surface in the vertical and horizontal states of the optical axis was reduced from 119 nm and 106 nm to 13.3 nm and 4.8 nm by optimizing the position of the mirror support point, the position of the triangular plate flexure joint, and the key dimension parameters of the support system’s flexible parts; under the state of 1 °C temperature difference, the specular aberration is reduced from 2.8 nm to 1.9 nm; the first-order resonance frequency is increased from 80 Hz to 130 Hz. Finally, this method is used to verify the correction ability of the semi-active support system. The results show that the correction rate of the semi-active support system for mirror defocus, primary astigmatism, primary coma and primary spherical aberration can reach up to 99%. The amplitude of each aberration is less than 1 nm; the correction rate of the RMS value of the mirror’s surface shape can reach up to 46.5% under it′s own weight state at room temperature, and the correction rate is 31.28%.