This paper discusses dynamic behavior of a special transmission system—soft web winding system. Soft web such as film acts not only as the wound object but also as transmission parts in the roll-to-roll systems mentioned above. Its longitudinal and transverse vibration have great effect on the film printing quality or corona treatment quality before wound , on the wound roll’s quality and even on the whole performance of system. Based on Hamilton’s principle, a dynamic mode is established by elasticity theory. Method for calculating natural frequency of moving film web and related resonating condition is adopted, which provides a feasible way for the optimization of structural design for web winder.