The torsional vibration model of main drive system for 6-high cold rolling mill was established. Calculation formula of torque amplification factor (TAF) of main drive system was put forward when the clearance exists or not. The TAF-curve, changing by the biting time of blank, was drawn, and whether torsional vibration could cause mechanical failure in weak link of main drive system was judged. The TAF variation with the biting time of blank was analyzed when the clearance exists. The method to reduce TAF was proposed before the clearance was eliminated, and the effect caused by the impact on main drive system was alleviated.