The thermal deformation of the hydrostatic motorized spindle affects seriously machining precision,so it is necessary to analyze the effect factors of the thermal deformation of motorized spindle system. The process parameter and structural parameter are significant factors,such as the flow-rate of cooling water,the input pressure of bearing oil,the spindle speed and the bearing’s structural parameter. The results show that the maximum deformation of the motorized spindle system was in the place where the grinding wheel was installed. The variations of the flow-rate of cooling water and the input pressure of bearing oil affected the maximum deformation on the front surface of the spindle. When the flow-rate of cooling water or the input pressure of bearing oil was at a certain value,the thermal deformation was brought under effective control. If the flow-rate of cooling water or the input pressure of bearing oil increased more,the reduction of the thermal deformation was less. As the speed changed,the flow condition of the bearing oil was different. The maximum deformation of the spindle ’s front surface in the condition of turbulent flow was more than two times of that in laminar flow. The bearing’s length-to-diameter ratio and the length of bearing axial sealing edge were optimized,which minimized the maximum radical thermal deformation on the front surface of the spindle.