The technology of CCD as a star sensor starlight detector has been very mature, but the bottleneck is becoming exposed. On the basis of parameters of IBIS5-B-1300A, the article analyzes CMOS APS noise effects on position accuracy of star sensor, obtains the maximum star position accuracy which a CMOS APS sensor can achieve and verifies the theoretical analysis results through modeling and simulation. Then two conclusions are drawn. First, when gauss dispersion radius is 0.7 and the windows size is 5×5, we can obtain optimized subpixel accuracy; Second, Fixed Pattern Noise (FPN) has the greatest effect on the accuracy of centroid algorithm. This article provides a theoretical basis and support for the selection of the starlight detector for a star sensor system.