Backwash filter is mainly composed of high quality carbon steel cylinder, stainless steel wedge filter, butterfly valve and sewage device. When the filter is working, the butterfly valve is in the open state. The water flow enters the filter from the inlet and is filtered through the filter screen to the outlet. The dirt and impurities in the water are intercepted by the filter screen. When the strainer needs to drain, close the butterfly valve. Open the blowdown valve, the water is filtered through the first half of the filtration, part of the water flow directly into the system, the other part of the water flow from the outside of the filter into the inside, discharged through the sewage outlet. Play a reverse automatic washing screen and remove the role of dirt impurities. The whole recoil blowdown process does not need to shut down the system, which realizes the on-line blowdown.