During rolling the flatness is kept within tolerances by the flatness control system, the important objective of this work is to study about the flatness control and improve the quality of the rolling product in hot rolling mill. HAGC (hydraulic automatic gauge control system) is defined as a control system for line conditioning mill equipment to ensure the thickness of rolled products to meet the target tolerances. All the mill stand by (roughing mill and finishing mill) equipped with short reaction time, repeated the long-stroke and high precision positioning hydraulic cylinder (no pressure screw), strip thickness can achieve high accuracy. After using a long stroke hydraulic cylinder, can significantly improve control accuracy and dynamic response capabilities, and significantly reduce the length of strip thickness tolerance zone.