According to the requirements of high force-thermal stability and high performance of the near space ball-borne telescope, the design of the secondary mirror assembly was optimized. Although the near-space ball-borne telescope was not as harsh as the rocket launching mechanical environment, its unique flight process was affected by temperature changes and acceleration. At the same time, it had a strict quality requirement due to carry with balloon. Compared with the traditional mirror design method, the method of combining entity optimization and base structure optimization, integrated optimization was used to design the mirror, and introduced comprehensive evaluation factors to optimize the overall performance of the secondary mirror. The performance of the final secondary mirror assembly is good, indicating that the optimization method is effective. Through finite element simulation analysis, it is obtained that the secondary mirror assembly has a rigid body displacement of less than 3 μm, a surface accuracy better than λ/50 under the condition of gravity and temperature change of ±3 ℃. Under 0.02 mm assembly error, the shape accuracy is better than 1 nm. The first-order frequency of the secondary mirror assembly is 203.8 Hz. The 10 g acceleration stress response (35.4 MPa) is far less than the material yield stress. Using this method to optimize can obtain high force-thermal stability, high performance secondary mirror assembly.