According to the density of oil substances, they are generally smaller than water, and they can be divided into floatable oil, dispersed oil, emulsified oil and dissolved oil according to their existing state in water, among which floatable oil and dispersed oil have larger particle sizes and can be separated from water by virtue of the specific gravity difference between oil and water. The wastewater flows in from one end of the tank and flows through the tank at a relatively low flow rate. During the flow, the oil particles with a density lower than that of water rise to the water surface, and the water flows out from the other end of the tank. An oil collecting pipe is arranged at the upper part of the tank body to collect the floating oil and guide it out of the tank. Emulsified oil cannot be directly removed by static sedimentation, and it needs to be demulsified first and converted into floatable oil before it can be removed. Dissolved oil is dissolved in water and cannot be removed by a grease trap.