Abstract: Ball screw units is the key component of CNC machine tool. The thermal error caused by temperature rise during operation affects the positioning accuracy of machine tool. In this paper, the temperature rise and positioning error of X-axis of a CNC lathe are measured. Through theoretical analysis and calculation, the pre-stretching quantity and pre-stretching moment are determined, and the supporting structure of pre-stretching ball screw is designed. Finally, the temperature rise and positioning error are measured, and the workpiece processing test is carried out, which proves that the pre-stretching method is feasible.