The determination of optical system parameters is the first step in the design of optical system of star sensor. In this paper, the influence of the field of view, focal length and relative aperture of the optical system on the star sensor is analyzed. In the selected detector conditions, according to the field, the focal length, and the magnitude of threshold of mutual restriction, signal-to-noise ratio, the size of the catalog and optical system design considering the level of to determine the optical system of the field, focal length, relative aperture, magnitude of threshold, spectral range and center wavelength. In order to meet the signal to noise ratio, the stellar detection limit, detection probability and other requirements, the selected detector were calculated. The method of determining the parameters provide a reference for the design of the optical system of star sensor.