In the aperture synthetic radiometer system, the calibration of phase error plays an important role in improving the image quality. Errors of both antenna and receiving channel will cause aperture synthetic phase error in each sub-aperture, so arouse deterioration in image quality. An in-orbit external calibration method for aperture synthetic radiometer using external calibration sources on ground is proposed, mainly aiming at the calibration of the antenna pattern error and the antenna position error. The simulation shows that the method can correct these two errors well.