Abstract: In the cold rolling process in unsteady state, the set value of roll bending force is important to strip flatness control system. Based on both the theoretical analysis and practical applications, the mechanics and control method were applied to analyzing the roll bending force preset, as well as effects of rolling force, friction force and acceleration/deceleration on the roll bending force. A relevant mathematical model was therefore developed in reduced expression. The results indicated that the roll bending force can be readjusted by the influencing coefficient of rolling force on it, while the rolling speed will affect the roll bending force through friction force. This model has been applied successfully to the 1450 tandem mill for cold rolling and achieved good control effect, thus providing theoretically support for controlling the roll bending force in unsteady state.