An algorithm to improve the positioning accuracy of spot centroid for Shack-Hartmann wavefront sensor is proposed. The detection errors of spot centroid are analyzed. The image resolution is improved by utilizing the detection window whose size is matched with that of spot and using the interpolation method. The centroid position is calculated by the two-order moment algorithm. The proposed algorithm is used to process and calculate the spot images with noise and the wavefront reconstruction diagram of the test component is also presented. The results show that, compared with the traditional algorithm, the proposed algorithm improves the detection accuracy of centroid by about 0.8 times.