Measurement accuracy of star sensor is restricted by ambient temperature.This paper describes an effective approach to analyze thermal effects in optical systems of star sensor.The star position errors,which are caused by thermo-optic effects,are calculated in an optical system of middle-precision star sensor.The results show that the star position errors are not remarkable near field angle 0°.And material matching is an effective measure in reducing the star position errors caused by changes of ambient temperature.After thermal compensation design,the maximal error is only 0.02″ when the ambient temperature changes from-20℃ to 60℃,which is about 1/7 of the original optical system's maximal error.