A water storage tank is provided with a water storage bag for storing finished water and a space for accommodating squeezed water outside of the water storage bag. The water storage tank enables a rapid introduction of squeezed water by having a flow guiding groove connected in communication with the space for accommodating squeezed water. A hollow tube extending into the water storage bag is arranged to prevent distortion of the water storage bag, so as to completely squeeze the finished water out of the bag. A cylindrical middle part and two ellipsoidal or hemispherical ends of the water storage tank can improve the stress bearing performance of the water storage tank. A household water purifier using the water storage tank is also described.