The radial positioning accuracy and stiffness are two important indexes to measure the performance of tool system. Once HSK tool system is in operation, the gap between the spindle and shank will enlarge, thus will make the radial positioning accuracy and stiffness lower. The influence factor leading to this problem is analyzed in this paper through elastic mechanics and finite element analysis. It can get from the analysis that the double-position structure and certain amount of interface are key factors to keep HSK high radial positioning accuracy and stiffness. In addition, the influence of clamping force and rotate speed to radial stiffness is presented that higher speed and larger clamping force make the radial stiffness better. Finally, the paper has verified the analysis of radial stiffness through the experimental measurement with different fit of HSK.