According to detection requirements of the star sensor system, a double Gauss optical system with image side telecentricity is designed. This optical system, not only satisfies the special requirements of dispersed spot and energy centralized distribution, but also possesses large aperture, low distortion, small color deviation, and insensitivity to defocus. The optical performance of the star sensor is quantitatively measured by modulation transfer function (MTF) analyzer. 80% energy of dispersion spot for each field of view is concentrated in a circle with 20 μm diameter. Meanwhile, a method using one-dimensional precision rotating platform and single star simulator to test the centroid distortion of optical system is proposed. The test results show that centroid distortion of the star point is better than 9 arc second.