The characteristics of wave conditions with large tidal ranges and small wave heights in China's coastal waters limit the operation time of traditional single-stage overtopping devices, while the multi-stage overtopping wave energy converters can increase the overall operation time of the device by distributing the upper and lower reservoirs. To study the overtopping performance of the multi-stage overtopping wave energy converter under real sea conditions, a two-dimensional numerical model of the device is developed and verified by physical model tests. The effects of the lower reservoir opening width, slope angle combination, and slope inundation length on the overtopping performance are investigated under regular wave conditions. It is found that a smaller opening width and two 30° slope angles improve the overtopping performance of the device, while the slope inundation depth has less effect. Further, based on the full-scale prototype device, the database of overtopping in a complete tidal cycle (12h) were constructed under real sea conditions in different seasons to provide the basic data for the Wave-to-wire model of power generation simulation.