Abstract: The machining precision and cutting efficiency of a numerical control machine tool to a great extent depend on the dynamic characteristics of the motorized spindle. To overcome the weakness of predicting the rationality of axis-bearing unit designed by traditional method, the dynamic characteristics of the motorized spindle installed in a certain high-speed horizontal machining center were studied in this paper. The results showed that the rotary speed designed was far lower than the critical rotary speed which matched with the low-order modal inherence frequency (659. 90Hz) of the spindle, so resonance vibration could be effectively avoided. If the spindle unit generated resonance vibration under the action of a periodic exciting forces, the stress of bearing 3 would reach the maximal value, and the huge vibration intensity would result in the loss of bearing working precision and even destroy the bearing.