Large modern hot strip mill is a complex electromechanical system.Vibration of rolling mill is a serious problem which effect the stability of production. By using torque telemetry system to test the torsional vibration of mechanical drive system，it is found that two typical characteristics of the strong or weak vibration exist in the waveform and spectrum of torsional vibration,meanwhile, there are harmonics in motor current and rolling force during rolling mill production, and when harmonic frequency is close to the natural frequencies of main drive system, rolling mill present a heavier torsional vibration on mill main drive system. For the study of its mechanism ,using computer simulation , the correlation of main drive torsional vibration and harmonic excitation by motor current and rolling force is solved. Research shows that with the harmonics excitation change, torsional vibration presents polymorphic variation strong or weakly on main drive system,which lay a theoretical foundation for vibration suppression.