For frequency fluctuation of AC power grid caused by the continuous increasing of distributed generation permeability, a design method of cascaded energy storage device based on isolated DC/DC converter is proposed. Combined with the maximum level condition of carrier phase-shift modulation and the power transmission limit of isolated converter, the design method of bi-directional power regulation region is described in detail. The general model of maximum discharge time is established, and the discharge time characteristics of energy storage device based on double active bridge and double half bridge isolated converter are analyzed in turn. The relationship between discharge time and converter type, transformation ratio, leakage inductance and supercapacitor quantity is obtained, which shows that the isolated converter optimization benefits to leakage inductance integration, easier manufacture of isolation transformer and reduce energy storage cost under the fixed discharge time index. Finally, the simulation results show that the proposed method has high control accuracy when the size, type and direction of power instructions change suddenly, which is helpful for the output power of device to follow the AGC instructions in real time.