The stellar detection limit(SDL) of image sensor is a key parameter of star tracker. It plays an important role in the selection of guide stars and the star pattern identification. In order to calculate the SDL of CMOS active pixel sensor (APS), the model of star influx and the noise model of APS are presented. The main noise sources of APS are analyzed. If SNR and total noise of APS are given, the SDL of APS can be calculated based on the typical response of 0 magnitude visual star per second per mm2. As an example, a specific SDL of APS is provided with given optical parameters and exposure time.