The wave overtopping performance of a circular ramp wave energy convertor is analyzed based on the 3D numerical simulation technology used for calculating fluid dynamics. By building up a three-dimensional numerical wave flume used for the VOF (Volume of Fluid) model of water-gas two-phase, a study on three-dimensional numerical simulation of this convertor is carried out. By comparing the results from the numerical calculations and the physical tests, the reliability of the established 3D numerical wave flume is verified. The optimal structure parameters of the device are determined by investigating the influences of the slope number of diversion blades and freeboard height of the device on the overtopping performance. On the basis of all these, a backflow plate is added to the edge of the slope surface of the device to reduce the wave reflection, by which the optimal design of the circular ramp wave energy convertor is studied. The optimal length of the backflow plate is explored by studying the effect of the backflow plate length on the overtopping performance of the convertor.