For the merit of highspeed onoff valve which are accurate control flow, low price, strong antipollution, high repeatable accuracy and good stability, the idea of using highspeed onoff valve instead of servo valve in automatic gauge control (AGC) system of rolling mill was presented. The highspeed onoff valve position control mathematic model was established based on pulse width modulation（PWM） control method of linear transformation of duty ratio in automatic gain control (AGC) hydraulic system, and its response curve of displacement, flow and rolling force were analyzed in the Simulink software when the cylinder had downward movement. The research results show that the high speed onoff valve can realize fast and accurate control of the cylinder block in AGC system, and the idea of using it instead of servo valve in AGC system is feasible.