The invention discloses a backwashing device for a membrane filter. The backwashing device comprises a rectangular shell, wherein the interior of the shell is separated into a filtering chamber and two collecting chambers by filter membranes; the filtering chamber and the collecting chambers all have rectangular structures; the two sides of the filtering chamber are adjacent to the collecting chamber through the filter membranes; netted filter plates are arranged on two side walls of the filter membranes; washing plates are arranged on side walls in the collecting chambers; the boards of the washing plates are parallel to the boards of the filter membranes; washing pipes are uniformly distributed on the washing plates; nozzles are distributed on the washing pipes; the nozzles face to the filter membranes; a washing branch pipe is arranged on each of the washing plates; one end of the washing branch pipe is communicated with each washing plate while the other end is communicated with a main washing pipe; solenoid valves are arranged on the washing branch pipe and the main washing pipe. The filter membranes can be backwashed at fixed period; matters blocking the filter membranes can be removed; the device can normally run, so that the quality of the end product can be guaranteed.