Balance for high speed motorized spindle is most important, it will influence the dynamic behavior of the high speed machine tools. In this paper, the GD-IV high speed spindle is introduced. In order to improve its balance accuracy, the step interference fit is developed to connect the rotor and the shaft. The interference fitted assembly has been modeled theoretically, the analysis highlights that the tolerance of the interference fit consists of the static section and dynamic section, the static section is determined by the transmitting torque while the dynamic section is determined by the centrifugal force. The Calculation of interference fit for the GD-IV spindle shows that the dynamic section is about 4.5 times larger than the static. Furthermore, the three dimensional model of the step interference fit between the shaft and the rotor has also been built up with finite element method and the stress distribution on the mating surface has been calculated. The results show that the maximum stress occurring near the chamfer region of the internal surface of the rotor is up to 235 MPa lower than the permissible material stress 278 MPa, so that the design of the step interference fit is reliability and safety.