A spot detection method was proposed for acquisition, pointing and tracking system in the case of azimuth offset and pitch errors exist at the same time. The position error between the broken spot and the Gaussian complete spot was analyzed. The problem was solved when the beam was drifted and the incident beam was not parallel to the line of sight of the antenna and the four-quadrant detector could not be detected separately. In the end,conduct experiments to verify the validity of approaches.The improvement of detection precision is also presented.