A star sensor is a highly accurate attitude measurement device, but it is susceptible to the thermal environment. Moreover, it is difficult to establish an optical-machine-thermo model of a high-precision star sensor by simulation. Accordingly, an experimental analysis method is proposed to simulate on-orbit thermal environment of the star sensor by heating the mounting surface and baffle in a vacuum tank and simulate the star using static star simulator. The triaxial thermostability of the star sensor can be evaluated by observing its output. A prism mounted on a mounting surface was used to remove the thermal deformation of the mounting surface by auto-collimation with an error margin of 4.5%. Results show that when the baffle is heated from 27.3 ℃ to 110.6 ℃, the drifts of the x, y, and z axis are 2.9″, 1.2″, and 2.6″, respectively. When the temperature control accuracy of the mounting bracket is (20±0.3) ℃, the optical axis drift is ±0.18″, which meets the requirements of thermostability indicators of the high-precision star sensor.