The stiffness of the spindle system refers to the ability of the shaft to resist positional changes at the working end (the midpoint of the width of the grinding wheel) under external loads (grinding force and belt tension, etc.). It is one of the important properties of the spindle components. The liquid dynamic and static internal grinding tool mostly adopts the structure shown in Fig. 1. The thrust shoulder of the structure is at the front end, and the axial deformation is minimal. Before installation, the front and rear bearings 3 and 5 are installed first, and the coaxiality is easy to ensure. However, due to the small spindle journal d, the stiffness of the system is increased. To this end, the structure shown in Figure 2 is used to improve its performance.