As the diameter of ground-based telescopes increases， the structural size of the telescope also increases accordingly， and the effect of the telescope's optical axis pointing caused by gravity becomes increasingly pronounced. The measurement and calibration of optical axis changes for ground-based large-aperture telescopes is a necessary prerequisite to obtaining high-precision orbital target data. To realize the estimation， measurement， and active correction of the telescope pointing error， the optical axis changing rules of the telescope system are analyzed from the structure of the telescope's opt-mechanical system， and an optical axis change measurement and calibration method of the is proposed. The experimental results show that the maximum relative change of the optical axis is 126″ when the telescope is at low elevation. The main factor causing the pointing change is the effect of gravity on the telescope truss structure. Finally， a calibration model based on the secondary mirror adjustment structure and the coma-free theory of the Cassegrain system is proposed to improve the tracking performance of the 4-meter aperture telescope. The maximum relative change in the optical axis of the telescope after calibration is within 3″. According to the test results， the ground-based large telescope pointing change measurement and calibration method can be applied to the calibration and mounting process of the telescope.