Abstract: During the long-term service, the servo axis of CNC machine tools has gradually produced wear and thermal deformation, which has a bad influence on the accuracy retentivity and precision stability of the servo axis.Among them, the preload of the screw-nut pairs can not only represent the wear of the contact surface between the ball and the raceway, but also indirectly affect the positioning accuracy of the servo axis, it is an important performance index of the screw-nut pairs.Therefore, in order to monitor and control the preload of the screw-nut pairs in real time, analyze functional requirements of active control, design the special structure of the screw-nut pairs based on the "monitor-actuator", analyze the stiffness characteristics of the screw-nut pairs system, build the active control test device of the preload of the screw-nut pairs, and develop the supporting software and hardware system.This platform can provide a foundation for the research on the accuracy retentivity and precision stability of servo axis of machine tools.