Star sensors are easily disturbed by stray light when working in the space environment: the quality of star image can be seriously reduced and the difficulty of star image preprocessing can be greatly increased. In this paper we propose a novel method for eliminating stray light interference based on the grayscale difference of star images. Firstly, since the grayscale difference between the edge of a star spot and the surrounding background pixels was very large, a grayscale forward difference was performed on the star image row by row. Based on the equivalent difference star image obtained, we could propose an edge pixel criterion for the star spot. Secondly, Stray light interference was eliminated from the star image through a connected component analysis based on the edge pixels criterion for the star spot. The relatively complete star spot areas were marked on the marker map. The simulation results demonstrate that the star spot extraction rate achieved through the proposed method was much higher than that obtained through traditional star image preprocessing methods. As long as the star spot is not submerged by the stray light, the proposed method is robust enough to ensure a successful extraction of the star spot.