In this paper, the centroid localization error of a star sensor with regard to the radius of defocus image spot and noise intensity is analyzed. The functional model for the optimal Gaussian radius about the two variables, standard deviations of centroid localization error, and noise intensity are established. Under the condition that the centroid localization precision is determined, the one-variable-function relationship between the optimal radius value of defocused Gaussian image spot and the background gray-noise intensity can be obtained. The simulation experimental results show that under the given centroid localization precision, the relationship between the optimal Gaussian radius and the noise intensity follows a functional curve, such as a parabola. After the noise intensity is determined by statistics, the optimal Gaussian radius of image spot can be obtained.