According to in-depth analysis of the interface between HSK shank and the spindle, the dynamic characteristic of HSK tool system in high speed machining is studied and numerical simulation is carried out with FEM. The HSK tool system deformation, contact stress and the change law of dynamic stiffness were opened out systematically. The relationship between the dynamic stiffness and loading capacity was given. The rotational speed per minute when HSK taper coordination position fails can be named limit maximum rotational speed Nmax of HSK. This provides the foundation for the design of the high speed machine tool.