An improved centroid algorithm was put forward considering the actual pixel photosensitive area shape and pixel fill factor of image sensor, which is based on the characteristic of different pixel photosensitive area shape and pixel fill factor with different coordinate values of the photosensitive area center. The locating accuracy was improved by modifying the values of pixel coordinates in traditional centroid algorithm. How to simulate a star point was described by analyzing gray scale distribution characteristic. The effectiveness of the improved centroid algorithm was verified by simulation. The simulation results show that the locating accuracy of the improved centroid algorithm is superior to the bilinear interpolation centroid algorithm and the traditional centroid algorithm both in ideal and noise situations.