A soil pressure balance shield gas preventing and controlling device effectively reduces a risk of tunnel gas explosion caused by overflow of a large amount of gas at a slag outlet of a spiral conveyer. The device comprises a high-sensitivity gas sensor. The high-sensitivity gas sensor is fixedly mounted at the side upper part of the slag outlet of the spiral conveyor, thereby detecting the gas concentration value at the position and conveying to a control system which controls the gate state of the spiral conveyor. When the gas concentration value reaches 0.05%, the control system automatically closes the gate of the spiral conveyor, thereby closing a gas entering channel of the gas into the tunnel.