In order to determine the ability of thermal energy storage of the steam accumulator, a new parameter, the volume-efficiency(VE), is proposed to determine the optimum design of the inner structure of the steam accumulator. This paper gives the definition of VE. The values of VE for series of inner structure sizes are obtained based on the temperature-field and the pressure in the steam accumulator. Thus the relation between VE and the diameter of tubular guide is obtained. The tendency of VE vs. the height of nozzles can also be available. An equation of VE is presented as function of diameter of the tubular guide as well as the height of the nozzles. From this equation, the optimum design of the 100m3 vertical steam accumulator is gained. The VE value for the real one standing in Shanghai Mineral Water Plant is obtained. The result has good agreement with its operation.