Abstract: The disturbances generated from axial flexible vibration of the high speed rotor of the inertia actuator will have a bad effect on the precision and the stability of the spacecrate attitude control. The model of the flexible vibration must be set up to decrease the disturbances of the high speed rotor. The spoke of the rotor can be regarded as the cantilever beam with a fixed end and a lumped mass end. Thus the model of the flexible vibration was obtained,and the characteristics of the flexible vibration of the rotor were analyzed. The results show that the violent flexible vibration will happen if the motivation frequency equals to the natural frequency of the rotor. Moreover,the energy method can be used to increase the precision of the model.