A device that separates oil, gas and water in formation fluid on the ground and accurately measures its output. It is divided into vertical, horizontal and spherical forms. For the convenience of transportation, horizontal separators are usually used for production measurement. The internal structure of a typical horizontal three-phase separator mainly includes: inlet diverter, defoamer, coalescer, vortex eliminator, demister, etc. When the formation fluid enters the three-phase separator, it first meets the inlet diverter, so that the liquid and gas are initially separated. After the gas with a large number of droplets is further separated by the coalescer, it is further purified by the defoamer and demister, so that it becomes dry gas and is discharged from the gas outlet. An air control valve is arranged on the exhaust line to control the gas discharge to maintain the required pressure in the container. Under the action of gravity, due to the density difference between oil and water, free water sinks to the bottom of the container, oil floats to the top, and enters the oil chamber over the oil-water baffle. The float-type liquid level regulator controls the discharge of crude oil by manipulating the oil drain valve to keep the oil level stable. The separated free water is discharged through the drain valve operated by the oil-water interface regulator to keep the oil-water interface stable.