The invention provides a pneumatic control overflow system for a mobile screw compressor, which ensures that the deflation amount can meet the requirement when the gas consumption is reduced, so that the rotating speed of a diesel engine and the internal pressure in a unit can quickly tend to be stable, the state of a safety valve can be stable, and the stable of the compressor can be stable. The system comprises a gas inlet controller, a stabilizing regulator, a speed regulator of the diesel engine and an oil-gas separator, and is characterized by comprising an overflow valve and a joint body, wherein the overflow valve comprises a piston cavity and a valve plug cavity; the oil-gas separator is connected with a gas inlet of the gas inlet controller; the stabilizing regulator is arranged from the gas inlet to the gas outlet of the gas inlet controller; the gas outlet is provided with the joint body; one path of the joint body is led to the speed regulator of the diesel engine, and the other path of the joint body is led to the inlet of the piston cavity of the overflow valve; the oil-gas separator is connected with the inlet of the valve plug cavity of the overflow valve; and the outlet of the valve plug cavity is communicated with a gas suction cavity of the gas inlet controller.