Abstract: For calculating out conveniently the average power of the vertical oscillating float of a wave-energy power generation device, the existing methods for design calculation are investigated. By solving the wave force based on the wave theory and the Froude-Krylov assumption method and by using the forced vibration theory of single-degree-of-freedom damped system to take wave excitation force as the input power to the vertical oscillating float and to take electromagnetic resistance of the generator as the main damping force, a mass-spring-damped vibration mechanics model is established and a computation method of the vertical oscillating float response is determined. And then, a computational example is given for the vertical oscillating float PTO system of an actual wave energy power generating device. On the basis of this example, the assumed conditions are expanded, and three-dimensional curves of the average power under different parameters are plotted and a table of the average power values is output by using the frequency response curve and the MATLAB tool. All these have clearly shown the status of the system response. This study has indicated that at a given ratio of wave height to frequency the average mechanical power of the PTO system has a peak value with the changes in the floating body mass and the damping torque, and the design of the parameters of the device around the peak area can achieve a more ideal effect. This modeling and calculation method can provide a reference for the optimal design of practical engineering.