Abstract: Aiming at the problem that the dynamic characteristics of the motorized spindle of a horizontal five-axis CNC machining center are poor and the vibration amount is large during the test process, the dynamic model is established, and the modal analysis is carried out by using ANSYS Workbench to obtain the natural frequency and modal shape. Based on the simulation results and the structural principle of the motorized spindle, it is diagnosed that the rear end of the motorized spindle has a large amount of overhang, and the gap between the external connecting flange and the rear bearing cover of the spindle and the cooling water jacket is large, resulting in low natural frequency and poor dynamic characteristics of the machine tool. For the above reasons, the auxiliary support optimization design is added to the rear end of the motorized spindle, and the modal simulation analysis is carried out. The feasibility is verified by comparing the natural frequency and critical speed data before and after optimization. The optimization design is applied to practice, and the vibration test of the motorized spindle is carried out. The test shows that the vibration of the motorized spindle is greatly reduced, and the optimization design is reasonable. The research results provide optimization ideas and design methods for improving the dynamic characteristics of the motorized spindle system of the horizontal five-axis CNC machine tool.