This study aims to search the optimal system structure for a specific type of oscillatory wave-energy acquisition system, which owns the highest efficiency in acquiring the energy from sea wave. The system is mainly consisted of a float, a generator and several oscillators that are connected to the float or to each other by the elastic springs. In addition, all the components are capsulated in the float. For the purpose of comparison, when aided with an active control, the acquiring efficiency of the system with a near optimal structure is analyzed. Under the assumption of random sea wave, three steps are adopted to investigate the optimal system structure. First, the mathematical model of the capsulated float that can acquire the maxima power from sea wave is derived by the spectrum analysis. They offer the messages that what the dynamic properties of an optimal structure should be in order to acquire the maxima power. Second, the dynamic properties of the general system are analyzed. It is to examine the effect of increasing system¡¦s degrees of freedom in offering the flexibility of varying system dynamics to match the desired ones. The limitation of increasing the degrees of freedom is especially examined. Then, the maxima attainable powers for the systems with different degrees of freedom are simulated by the genetic algorithm. It is to support the inference made from the analysis about the effect of increasing the system¡¦s degrees of freedom. Finally, a preliminary examination of the effect of active control in power acquisition is done. The study indicates that an oscillatory system structure with two degrees of freedom is a near optimal structure for energy acquisition. An increase of system¡¦s degree of freedom shows little effect in improving system¡¦s dynamic characteristics in the main frequency range of sea wave. The proposed active control scheme is shown to be effective in improving the system¡¦s dynamic characteristics to enhance the energy acquisition from sea wave. However, the extra-energy consumed in the control action makes the increase of net energy acquisition negligible.