In this paper, computational fluid dynamics (CFD) simulations are carried out for the purpose of finding the optimal design parameters of a spiral-reef overtopping device. In order to maximize the overtopping flow rate, geometrical parameters of the device were systematically examined in numerical computations. In all simulations, the commercial CFD program FLOW3D was used. In this study, regular waves with a period range of 4~6 seconds (which are very common in the Korean southern sea) are considered. In the first phase of the study, two-dimensional parameters including device ramp angle, ramp shape and draft were investigated. Fully three-dimensional CFD simulations were then conducted to understand wave flow over the device and the guide-vane effect on overtopping on it. The calculation results show different overtopping processes between 2D and 3D simulations, and optimal design parameters were identified based on numerical results. These findings can be incorporated in the design of an overtopping device to obtain better overtopping performance.