Abstract: A water saving tank includes a tank box and a supplementary device. The tank box includes an inlet, a supplementary port and an outlet. The inlet collects the water from a shower head, the supplementary port connects to an exterior water source, and the outlet connects to a fill device in a cistern of a water closet. A position level of the tank box is higher than that of the cistern of the water closet. The supplementary device joints to the supplementary port; as the water in the tank box drops to a first predetermined water level, the water of the exterior water source flows into the tank box; as the water in the tank box is filled up to a second predetermined water level, the water of the exterior water source is blocked to flow into the tank box. The water is saved effective thereby and further is able to be reserved for spared.