A star sensor thermal drift calibration system based on error decoupling is proposed. According to the calibration principle and the simulation requirements of the star, a self-collimation optical calibration system and an error decoupling optical path are designed. According to the characteristics of common optical path, the installation deformation error of the star sensor and the coupling error caused by the thermal deformation and the installation deformation are obtained by the autocollimator and the CCD, removing the installation deformation error from the coupling error to obtain accurate star sensor attitude shift quaternion. The simulation results show that the maximum variation errors of the star sensor around the axes are 0.2638, 0.1317, 0.0472 (″)/℃ in the environment range of -25~60 ℃, respectively, and the errors are controlled in the range of 0.02 (″)/℃ with the ideal result. This provides a new idea and method to reduce the coupling error generated in the calibration of star sensors and improve the accuracy of thermal drift calibration system.