The invention relates to a triphase flow metering device of an oil well and needs to overcome the problems of bad metering effect and environmental pollution. The triphase flow metering device is formed in a way that a separation control tank, a mass flowmeter, a gas flowmeter, a gas-liquid buffer tank, a liquid valve, a gas valve and a check valve are connected with each other by flanges and pipelines, and is characterized in that a gas-liquid separator with annular ladder drop type is arranged at the upper middle part of the separation control tank, the middle part of the separation controltank is provided with a slideway rod, and the auxiliary valve of a gas-liquid control valve is arranged on the slideway; the top of the separation control tank is connected with a gas outlet conduit,a gas conduit is provided with a gas outlet valve, a gas flowmeter with the temperature and pressure compensation, a gas flow regulating valve and a gas-liquid buffer tank; the liquid outlet conduit of the separation control tank is provided with a liquid outlet valve, a mass flowmeter with the temperature and pressure compensation and a liquid flow regulating valve; the conduits are combined with gas conduits at the lower part of the gas-liquid buffer tank to form a header, and the header is connected with production pipelines. The triphase flow metering device is applicable to on-line metering with automatic phase splitting of oil, gas and water.