Abstract: Based on the dynamic characteristics of the spindle unit of VDF850 machine tool, the spring was used to simulate the elastic support of the spindle-bearing interface, and the finite element model for the dynamic analysis of the spindle unit was established. The modal and harmonic response of the spindle were analyzed with the finite element simulation software ANSYS. The mapping relationship between natural frequency and pre-tightening force of the spindle was studied, and the frequency response characteristics of spindle unit excited by cutting force were obtained. The results show that the natural frequency of the spindle unit increases slightly with the increase of preload, the first critical speed of the spindle is much higher than the maximum speed of 8 000 r/min, and the dynamic characteristics of spindle unit are good.