Accurate calibration of focal length deviation is the key to ensure that the star sensor outputs high-precision attitude information, and optical distortion is the primary source of focal length deviation.According to the geometric model of star sensor imaging and the optical distortion model, the mathematical model describing the coupling relationship between optical distortion and focal length deviation is established, and the expression describing the relationship between the optical distortion and the equivalent focal length deviation is derived.The reference distortion threshold δrmax of navigation star point is proposed for focal length deviation calibration.The simulation results show that the calibration accuracy of focal length deviation when selecting the star points that are all within the distortion threshold is 5 to 10 times higher than selecting those that are partly or all outside the distortion threshold, which reduces the influence of optical distortion on calibration accuracy of focal length deviation to a great extent.