Fine guidance sensor (FGS) in the space astronomical telescope can realize high-precision measurement of attitude information for the precision image stabilization system. The imaging effect of the CMOS image sensor (CIS) directly affects the calculation accuracy of FGS. However, the CMOS imaging device has the best readout range in actual operation, and the linearity of the incident ray intensity and the number of photo-generated electrons beyond this range is low, and the effective star points cannot be obtained to meet the centroid coordinate solution of the later stage. In order to solve this problem, a method for estimating the best integration time of different magnitude is proposed and the integration time can be used as the main basis for the selection and adjustment of detector parameters. The measurement results show that both the dark current of the sensor and the RMS noise will increase with the increasing integration time. According to the star facula distribution model, the best integration time range of commonly used stars is calculated. Combining with the star spot distribution, it is concluded that there are about seven stars in the field of view of 7-magnitude stars, which demonstrates the detection ability of small CMOS devices for star spots.