The present invention relates to a gas constant-flow output feedforward control system, which comprises a gas storage tank, an operation station, a PLC controller, a regulating valve, a stop valve and a pressure sensor, wherein the pressure sensor is arranged inside the gas storage tank, the output pipeline of the gas storage tank is sequentially provided with the regulating valve and the stop valve, and the PLC controller is connected with the operation station, the gas storage tank and the regulating valve. The method comprises that: the PLC controller detects the initial tank pressure and the temperature of the gas storage tank, and controls the regulating valve according to the control information transmitted by the operation station, and the PLC controller controls the stop valve to close when the gas output is not required. According to the present invention, the structure is simple, the control method is relatively simple and easy to perform, and the gas with the constant mol / s-grade flow can be output.