An external metering valve disposed on a gas fuel tank including an outlet rod is provided. The outlet rod includes an outlet orifice at a front end thereof. The external metering valve includes a metering valve and a gas guiding member. The metering valve includes a metering chamber. The gas guiding member includes a communication pipe, a blocking portion on the communication pipe, a discharging channel and an inlet orifice. The gas guiding member has a sealing outlet position and a metering outlet position. At the sealing outlet position, the outlet orifice outputs a gas and stores the gas in the metering chamber. At the metering outlet position, the blocking portion seals the outlet orifice through an airtight ring, and outputs the gas stored in the metering chamber through the inlet orifice and the discharging channel.