This paper concerns the response and efficiency of a two-body wave power conversion device. The physical problem of the device is simplified as a forced vibration system with viscous damping in two degrees of freedom. Based on the linear wave theory, wave excitation forces, added masses and damping are derived by use of an eigenfunction expansion matching method. The expressions of the response and efficiency are deduced from the motion equations of the device, and the optimal principle is presented as well. Numerical results indicate that damping optimal curve has a wave peak that is independent of the spring, and the value of the peak and the corresponding frequency are only related to the calculation conditions. Optimal curve presents two peaks when the spring exists, and the corresponding frequency width decreases with the increase of elastic coefficient. The damping is relative small at low frequency peak, while the response is relative high. The damping is relative high at high frequency optimization peak, while the response is relative small and almost smaller than the amplitude of the incident wave. With the decrease of the external damping coefficient, the relative motion amplitude and the efficiency increase, while the width of crest decreases.