Optical distortion is one of important error sources of Wide Field-of-View (WFOV) star sensor,which has a great effect on focal length calibration due to coupling between the errors.To solve the problem,we established a mathematical model to determine the coupling relationship between optical distortion and focal length based on pinhole model.The referring threshold βmax of guide star incident angle is deduced using least square method to calibrate focal length,which provides the theoretical reference for focal length calibration.Simulation results demonstrate that:compared with calibration accuracy in cases that partial or all the guide star incident angles are greater than βmax,the focal length calibration accuracy in case that all guide star incident angles are less than βmax is increased by five times and ten times respectively.The influence of optical distortion on focal length calibration is effectively reduced.