Pointing accuracy is a challenging and key technical indicator of large radio telescopes. When telescopes are working, with the change of azimuth and elevation, the comprehensive influence of gravity and sunshine on the legs of sub-reflector will cause displacements of sub-reflector of the telescopes. Thus the pointing error increases, and efficiency of antenna decreases. Based on the TianMa 65 meter radio telescope, a measurement system for sub-reflector's displacements is constructed by Position Sensing Device (PSD) method. Three dimensional displacements data of sub-reflector can be collected by this method. Gravity model is built. Results from this model and existing model built by radio method are compared, and their results are consistent. In addition, sub-reflector's displacements caused by sunshine (local temperature effect on legs) can be analyzed. And this system can monitor displacements of sub-reflector caused by wind load and instantaneous start-stop of telescope.