The invention relates to a fully automatic water-saving and storing device for a washing machine, and a working method thereof, which pertains to the technical field of water saving device. The devicecomprises a time program controller, an automatic purified water and sewage separating device, a sewage drainage area and a purified water storage area; the time program controller is connected witha circuit of the automatic purified water and sewage separating device that is provided with a water inlet, a sewage outlet and a purified water outlet; the sewage outlet is communicated with a drainage opening by a sewage draining channel; the purified water outlet is arranged in the purified water storage area and provided with a purification and filtering device at the bottom; the sewage drainage area is provided with a water outlet and communicated with the drainage opening by the sewage draining channel; the purified water storage area is internally provided with an inner drum and a wateroutlet communicated with the drainage opening; and an omni-directional wheel, a push-and-pull handle, a drainage hose fixing clamp, a water cleaning brush and a reflexibie cover are arranged on the device. The device realizes fully automatic water saving and storing under the control of programs. The device is matched with washing machines for use, has reasonable design, complete functions and convenient operation and can improve the utilization rate of water.