A piecewise nonlinear dynamic model of rolling mill is established which contains the sanction of hyardulic system. The characteristics of bifurcation of autonomy system and nonautonomy system are analyzed by means of sigular stability theory and singularity theory, respectivly, and different topological structure of solution are obtained under different parameters. At last, the numerical simulation of rolling mill exhibits many different motions such as periodic motion, period-doubling motion and chaos, etc. It found that the change of parameter influences the motion state of the system. The results supply theoretical bases for optimal control of rolling mill.