Aiming at the problem of optical system coordinated with carrier in practical application, the optical system was placed on the plane of horizontal coordinate system and the star image was taken. Coordinates of star image were calculated through image processing. The equatorial coordinates of stars were obtained by automatic celestial body identification and star apparent position calculation. Reference coordinate was established in plane of the sky centered by zenith point. The equatorial coordinates of stars were converted to reference coordinates, and then to topocentric image coordinates according to the relation between zenithal tangent plane and topocentric coordinate. The calibration equation between image coordinates and topocentric coordinates of stars was set up to calculate the deviation parameter of system. The calibration accuracy is arc-second magnitude. The high precision coordinate conversion from star sensor to carrier is realized by this method.