Deck structures, whose safety is threatened by catastrophic waves, are widely used in offshore and coastal engineering. This study aims to enhance the understanding of the impact of waves on deck structures through a combined approach of hydrodynamic experimentation and numerical simulations. In this study, the wave impact forces and pressure on a deck structure, which was simplified as a rectangular box model, were experimentally measured. The numerical model based on smoothed particle hydrodynamics was validated using experimental data due to its capacity to compute the wave force exerted on the box structure accurately. A characteristic velocity parameter was defined to explore the relationship between the wave-slamming forces and the wave velocity field. It was found that the characteristic velocity was highly correlated with the wave impact forces, which could be fitted using a negative exponential curve. In addition, the characteristic velocity can be roughly calculated using Stokes’ second wave theory, which offers a preliminary method for estimating wave impact loads on deck structures.