The invention provides a rolling mill AGC control system which comprises a feed-forward control device, a feedback control device, a second-flow control device and a hydraulic executing device, wherein the feed-forward control device, the feedback control device and the second-flow control device are respectively communicated with the hydraulic executing device; an inlet thickness tester and an inlet laser velocimeter are arranged at an inlet of a rolling mill, and an outlet thickness tester and an outlet laser velocimeter are arranged at an outlet of the rolling mill; detection signals of the inlet thickness tester, the inlet laser velocimeter and the inlet laser velocimeter are transmitted to the second-flow control device, signals of the inlet thickness tester and the outlet laser velocimeter are transmitted to the feed-forward control device, and signals of the outlet thickness tester and the outlet laser velocimeter are transmitted to the feedback control device. The invention provides a set of high-precision data tracking solution of the inlet signals and the outlet signals by combining the feed-forward control device, the second-flow control device and the feedback control device for AGC of the rolling mill, thereby achieving the aim of precisely controlling the thickness of band steel at the outlet of the rolling mill.