The three closed-loop control structure of CNC machine tool servo control system was given, and the control principle of each control loop was introduced. The specific test items, servo parameters and optimization methods of the speed loop, position loop and roundness optimization were given. According to the problems of three-axis vertical machining center, the single axis servo parameter optimization and roundness optimization were carried out, and the comparison of servo parameters and control performance before and after optimization was given. After optimization, the machining beat time of the three-axis vertical machining center is reduced by 2.3 s, and the machining error of the circular interpolation is reduced to less than 10 μm.