A metering valve includes a valve main body consisting of a body and a cover, a sliding rod and an elastic member. The body has a positioning hole, a metering chamber, a containing space, and two sealing pads disposed on two end portions of the metering chamber. The containing space is for assembling the gas cylinder. A groove is disconnected with the positioning hole, the metering chamber and the containing space. The sliding rod has two ends, wherein a gas channel is formed from one end toward an inner of the sliding rod, the sliding rod has a flange and a through hole, the through hole under the flange is for connecting the gas channel, and the sliding rod blocks a gas from the metering chamber by cooperating with the sealing pads. The elastic member in the groove is connected to the flange and the groove.