In order to analyze the reasons for star sensor performance degradation and the decrease of attitude measurement accuracy, performance degradation mechanisms of star sensor caused by Complementary Metal Oxide Semiconductor (CMOS) image sensor radiation damage were mainly studied. By establishing the correlation among the spatial radiation, radiation sensitive parameters of CMOS image sensor and the performance parameters of star sensor, the transmission mechanism from CMOS image sensor device parameters degradation to star sensor system parameters degradation was revealed. The ⁶⁰Co-γ irradiation test showed that the decrease of signal-to-noise ratio led to the decrease of star sensor detection sensitivity after irradiation, and the signal-to-noise ratio was the bridge between the CMOS image sensor and the star sensor system. The proton irradiation test showed that when the irradiation fluence was more than 3.68×10¹⁰ p/cm², the star point centroid could not be extracted correctly. The results lay a certain foundation for the research of star sensor attitude measurement error and correction technology, and also provide some theoretical basis for the design of high-precision star sensor.