Because of manufacturing and assembly errors in the geometric parameters of parallel robots, it is necessary to calibrate them to improve the positioning accuracy. In this paper, the object of the study is a 3-DOF secondary mirror of Giant Magellan Telescope, which is a 3-DOF parallel robot. In order to calibrate this mechanism, a kinematic model is developed and a least square algorithm is proposed. In order to supervise the tip-tilt movement of the 3-DOF secondary mirror directly and conveniently, an simulation model is also developed. Finally, through conducting some experiments, this calibration method is verified, and experimental results show a good effect.