Exposure time has effect on position accuracy of star sensor. In this paper,motion model of star spot which is combined with the effect of CCD noise has been established. The centroid accuracy of the star spot affected by sub-pixels arithmetic and CCD noise were analyzed at different exposure time with CCD47-20 chip,and the optimum exposure time was calculated. The results show that the accuracy of arithmetic was degraded and the accuracy of star spot affected by CCD noise was decreased when the exposure time increased. The optimum exposure time is 200 ms when the motion rate is 4.67 pixels per second for a single star of 6.5 magnitude,and the star spot location accuracy is better than 1/50 pixel with 5×5 window.