Detection sensitivity is an important parameter of star tracker, which can express detecting limit of the weakest detected visual star magnitude. In this paper, the expressions of APS star detected signal and APS noise are given, so signal-noise ratio (SNR) of star tracker can be obtained. And the detection sensitivity model is obtained from SNR criterion of star target detected, star detecting probability is analyzed. Moreover the evaluation modal of the detection sensitivity in star system is deducted. The effects of detection sensitivity of APS star tracker were discussed.