Abstract: The mechanical equilibrium problem in shield tunneling is analytically analyzed. The equilibrium equation between thrust force of shield machine and tunneling resistance is proposed. Taking account of the coupled reaction between shield machine and soils, a new mechanism model for controlling the earth pressure acting on the headchamber is presented. The optimal procure is developed for controlling the earth pressure acting on the headchamber. The numerical simulation shows that the controlling time will take about 20 s while the rotation speed of screw conveyor is singly adjusted, and the controlling time will take about 10 s while the rotation speed of screw conveyor and thrust force are simultaneously adjusted. The effectiveness and preciseness of the proposed model are verified by numerical results.