We studied the energy storage device of an electric vehicle for solving the low specific-power problem of its battery. Considering the characteristics and the control target of a hybrid energy storage device, we developed a five-operation mode, and proposed a fuzzy control strategy based on the vehicle speed and current constraints. To further improve the vehicle performance and reduce the output current of the battery, we applied linear optimization to the parameter design of the device. Simulation results show that the use of the hybrid energy storage device reduces the acceleration time of the electric vehicle by 12%, increases the braking energy recovery efficiency by 39%, and reduces the 100 km energy consumption by 8.55%. Moreover, it greatly decreases the output current of the battery, and thus benefits the cycle life of the battery and extends the driving range.