The invention discloses an elastic membrane type horizontal energy storage tank, comprising a tank body; a hot cavity outlet and a cold cavity outlet are formed in two ends of the tank body, respectively; a partition component is arranged in the middle of the tank body to divide the interior of the tank body into a hot cavity and a cold cavity, which do not communicate with each other; the hot cavity communicates with the hot cavity outlet, while the cold cavity communicates with the cold cavity outlet; and the partition cavity comprises an elastic membrane that can be deformed toward the hot cavity outlet or the cold cavity outlet. Water flow is led into the cold cavity or the hot cavity by means of a cold storage pump or a cold discharge pump, and with continuous working of a water pump, between the cold cavity and the hot cavity, the cavity on one side gets bigger continuously, while the cavity on the other side gets smaller continuously, and the elastic membrane is stretched toward the side where the cavity gets smaller; due to the existence of the elastic membrane, cold and hot water can be prevented from being mixed with each other, and the efficiency of cold storage and cold discharge is improved; moreover, the elastic membrane is passively deformed under the pressure of water to change the volumes of the two cavities without extra driving component, and therefore, the energy storage tank has the advantages of material saving, and high effective volume. The elastic membrane type horizontal energy storage tank is applicable to the field of energy storage water distribution.