Recently, the accuracy of hot strip gage is strictly demanded, and the strip thickness becomes the most important quality in hot strip rolling. It is well known that the accuracy of final strip thickness in the hot strip mill depends on the gage control performance at each stand. In order to improve the quality of the hot strip gage and reduce the strip thickness deviation , the Automatic Gage Control(A.G.C) system now is widely used in modern hot strip mills of the world. In this paper, the principle of feedforward control strategy in the AGC system is discussed and it¡¦s control performance is deeply analyzed. Besides, based on the study of comprehensively mathematical model and the establishment of simulator, the mathematical analysis and the simulation result can clarify the influence of the control system on strip thickness.