A filter with a backwashing unit, comprising a filter cartridge, the filter cartridge being internally provided with a filter element, the filter cartridge being provided with a water inlet and a drain outlet in an inlet region of the filter element and being provided with a water outlet in an outlet region, characterized in that the filter further comprises a two-position three-way valve, a second valve and a third valve, the two-position three-way valve being provided with a port P1, a port A1 and a port B1, the second valve being provided with a port P2 and a port A2, and the third valve being provided with a port P3 and a port A3; the port P1 being connected with the water inlet, the port A2 being connected with the water outlet, the port A3 being connected with the drain outlet, the port A1 and the port P3 being connected with the inlet region of the filter element, and the port B1 and the port P2 being connected with the outlet region of the filter element; and a filtering channel being formed by the water inlet, the port P1, the port A1, the inlet region of the filter element, the filter element, the outlet region of the filter element, the port P2, the port A2 and the water outlet 4 when the port P1 communicates with the port A1, the port P2 communicates with the port A2 and the port P3 does not communicate with the port A3, and a backwashing channel being formed by the water inlet, the port P1, the port B1, the outlet region of the filter element, the filter element, the inlet region of the filter element, the port P3, the port A3 and the drain outlet 5 when the port P1 communicates with the port B1, the port P2 does not communicate with the port A2 and the port P3 communicates with the port A3; wherein the filter cartridge is provided with a cylindrical outlet passage, the port P2, the port A2, the port P3 and the port A3 are cross-shaped on the circumferential wall of the outlet passage, the port P2 and the port A2 are oppositely arranged, the port P3 and the port A3 are oppositely arranged, the outlet passage is internally provided with a rotatable valve element, and the valve element is provided with a communicating hole and baffles at two sides of the communicating hole; and the valve element is provided with a first station where the communicating hole communicates with the port P2 and the port A2, and the port P3 and the port A3 are shielded by the baffles, and a second station where the communicating hole communicates with the port P3 and the port A3, and the port P2 and the port A2 are shielded by the baffles; the filter cartridge is provided with an inlet passage, the port P1, the port A1 and the port B1 are arranged in the inlet passage, and the inlet passage is provided with a rotatable valve plate; and the valve plate is provided with a first position shielding the port B1 and a second position shielding the port A1, and the inlet passage and the valve plate form the two-position three-way valve; wherein the filter cartridge is a drum, the filter element is an annular element, the center of the drum is provided with a return pipe, the filter element is arranged outside the return pipe, the upper end of the return pipe communicates with the port B1 and the port P2, and the lower end of the return pipe communicates with the outlet region of the filter element; and a gap between the filter element and the filter cartridge is the inlet region of the filter element, and a gap between the filter element and the return pipe is the outlet region of the filter element; and the filter cartridge is provided with a roller at a position of the inlet region, the roller interferes with the filter element and is sunken into the filter element, and the roller is capable of driving the filter element to rotate around the return pipe and the filter cartridge is also provided with a motor driving the roller to rotate; and the motor is configured to stop when the filter is located at the filtering station, and the motor is configured to start when the filter is located at the backwashing station.