The stiffness of the HSK tool system directly influences the efficiency and the quality of high-speed machining. Based on the mechanics of materials and finite element method, theoretical analysis and digital simulation are done with the stiffness of the HSK tool system. An equation to calculate the deformation angle of the HSK shank is proposed. The basic change law of the stiffness of the HSK tool system is shown and it offers theoretical base for properly applying the HSK tool system to maximize benefit. It is important in theory and application area to develop new-style tool system with our own intellectual property.