The rotor of high-Speed motorized spindles with eccentric mass generates two loads that are unbalanced magnetic pull and centrifugal force. Based on electromagnetic theory, unbalanced magnetic pull model has been developed. And the amplitude and frequency characteristics of the two eccentric loads have been also analyzed theoretically. The rotor finite element model used Timoshenko beam theory is established, which includes bearing stiffness matrix and the two eccentric loads. The natural frequency and vibration mode are calculated from the rotor dynamical equations. The analysis of dynamic behaviors of the rotor affected by eccentric loads shows that the unbalanced magnetic pull and centrifugal force are the main excitation force. Finally, an overall experiment on a motorized spindle is carried out under several operating conditions. The good agreement between the theoretical results and the experimental data indicates that the two eccentric loads seriously influence dynamic characteristics of the rotor.