A countercurrent biological filter tank comprises a water tank, grid frames, filter materials, communicating pipes and a water pumping device. A pumping tank and filter tanks are delimited by partitions on one side of the water tank so that the partitions between the pumping tank and the filter tanks are lower than the height of the water tank. The grid frame and the filter materials are arranged in the filter tanks. The communicating pipes are communicated with the water tank and the filter tanks. The communicating pipes are respectively provided with switching valves and a drain pipe, so that the filter tank and the drain pipe can be communicated by switching the valve. The water in the pumping tank is delivered by a pumping device to the water tank, so that the water flow in the water tank naturally flows into the filter tank to be filtered by gravity and overflows to the pumping tank. By switching the switch valves and gravity of water, water in the filter tank is discharged from top to bottom, which is opposite to the water flow direction during the filtering to form a backwash water flow to effectively backwash and flush away impurities, dirt and the like attached to the filter material, thereby improving drawbacks that the filter material must be removed manually, and is inconvenience to clean and replace the filter structure in the prior art. Furthermore, the grid frame raises the filter material to forms gap and guide and disperse the water flow so as to make the filtering water flow or the backwash water flow smoother, more dispersed and activated when passing through, thereby enhancing the filtering and cleaning effect. This invention is environment friendly and is a biological filtration capable of avoiding affecting the population of nitrobacteria colonies due to the violent shaking of water quality caused by daily cleaning operation, so as to effectively extend the service life of the filter material, thereby improving the shortcoming of the convention
