The energy storage system is the key energy storage process of hydraulic power takeoff system in a floating wave energy converter(WEC). The components of an energy storage system which is in a floating WEC was introduced, and the principle stable power generation under the huge wave and 0-1 power generation under small wave were analyzed. A mathematical model about a couple of key design parameters including filling pressure, minimum working pressure, maximum working pressure and energy storage value was set up. Numerical simulations were carried on by calculating examples, and a large number of experimental data were obtained. The experiment results show that the lager the difference between the maximum and minimum pressure, and the energy is released in a single cycle, the lower the cost of the energy storage system is. The design safety value of the precharge pressure is 80% of minimum working pressure. There is a coupling relationship between the four key parameters, such as the energy storage value, the maximum working pressure, the minimum working pressure and the precharge pressure.