A device for preparing washing water, belonging to the technical field of electro-chemical and membrane filtration water treatment. The device comprises a water container which is respectively provided with a water inlet and a water outlet; at least one pair of a cathode and an anode are arranged within the water container; a water-permeable porous membrane is clamped between the coupled cathode and anode with no gaps, and the area of the inside of the water-permeable porous membrane opposite the cathode or the anode is smaller than the area of the inside of the cathode or the anode opposite the water-permeable porous membrane. The device can generate washing water which contains a large a number of ultra-micro bubbles and strong oxidation factors and has excellent reducibility.