Abstract: Based on the structure character and mission requirements of star sensors used by high detectivity satellites, thermal design of star sensor assembly was accomplished by combining thermal simulation analysis and thermal balance test. Firstly, the temperature target of the bracket of the star sensors was determined by the thermal transformation analysis. Then according to the orbit parameters and the positions of star sensors, the incident space heat fluxes of three star sensors were computed. Besides the fluxes, internal heat sources of star sensors and degradation of MLI were also considered to complete the design. Both the passive thermal control and the active thermal control were applied. The simulation analysis indicates that the temperatures of the assembly was between 17.0℃ to 19.1℃. Finally, the thermal design was examined by thermal balance test and temperature test in orbit. The results of thermal balance test show that the temperatures of the assembly are between 17.3℃ to 18.7℃ and the remote data show that the temperatures are between 16.0℃ to 19.1℃. Both the data indicate that the temperatures of the assembly are between 18℃3℃ and the thermal design is valid and reasonable which can meet the mission requirements.