The coupling performance of tool system is the important factor for the machining quality of high speed machining. Based on the deformation of contact surfaces of HSK tool system, the paper makes the academic analysis by elasticity theory and finite element simulation to high speed spindle/tool shank coupling system. The property of the contact stress and the radial deformation as well as the torsional stiffness with different amount of interference has been analyzed. In this method, the paper brings forward how the amount of interference will affect the coupling performance. Thus, it will give academic evidence to design and select tool system in the process of high speed machining.