The grease trap utilizes the different specific gravity of suspended solids and water in wastewater to achieve the purpose of separation. The grease trap structure is mostly advection type, oily wastewater through the water tank into the rectangular grease trap plane, along the horizontal direction of slow flow, in the flow of oil on the surface of the water, by the oil collection pipe or set in the surface of the pool scraper pushed to the oil collection pipe into the dewatering tank. Heavy oil and other impurities precipitated in the grease trap accumulate in the sludge hopper at the bottom of the tank and enter the sludge pipe through the sludge discharge pipe. After the grease trap treatment of wastewater is overflowed into the drainage channel discharged out of the pool for subsequent treatment to remove emulsified oil and other pollutants.