In order to discuss the dynamic behaviors of the bearingrotor system of high-speed motorized spindles, a rotor dynamic model with centrifugal force, gyroscopic couple and system damp was presented by using finite element method and Timoshenko beam theory. An overall model test on a 120MD60Y6-type motorized spindle was carried out to acquire the natural frequencies with different work conditions. The results indicate that centrifugal force will lead to the bearing softening and spindle shaft softening, and the latter is the main reason of the decrease of the natural frequency of the spindle system. Each natural frequency of the spindle system was divided into the forward and backward modes, and the forward and backward modes had different change regularities with speed. The good agreement between the theoretical results and the experimental data shows that the proposed model is capable of accurately predicting the dynamic behaviors of highspeed motorized spindles.