Abstract: In the main drive System of blooming mill the Self-excited vibration aroused by then slipping between the rolls and ingot is usually the main cause of first transceudence failure. The paper analyses the condition of self-excited vibration when there exist backlashes in the main drive system. A criterion is set up to identify whether the self-excited oscillation can be excited and a first order approximation for amplitude of steady self-exited oscillation is obtained. And also considers the influence of structural damping and normal pressure on torque amplification factor of self-excited oscillation. Mnorever analyses. The conditions of single face slipping and double face slipping.