Disclosed is a method for making a rubber liner of a composite pressure vessel. At first, rubber is located in a mold. The rubber and the mold are located in a vacuum bag, heated and pressed so that the rubber is vulcanized and molded into a semi-product of the rubber liner. Sand is filled in the semi-product. The sand is tamped and heated so that it is hardened and molded. A reinforcement element in the form of a wire or tape is wound around the semi-product. Two semi-products are brought into contact with each other. An un-vulcanized rubber band is provided around and attached to the semi-products. The rubber liners and the reinforcement elements are heated and vulcanized with composite resin. Finally, the water soluble sand mold is dissolved and removed so that the final product of the rubber liner is made, and so is the composite pressure vessel.