The calculation of the slamming pressure for flat-bottom and flat-bottom opening structures is the hot issue of the ship structural design. The water entry of the two-dimensional (2D) and three-dimensional (3D) rigid flat-bottom structures have been simulated by using MSC.dytran. The simulation results are compared with the published experimental results and the simplified calculation method in the specification, to analyze the influence of the 3D flow on the slamming pressure of the flat-bottom structure. The influence of the opening on the slamming pressure of the flat-bottom structure is also researched according to the simulation results of the water entry of the 3D rigid flat-bottom opening structure. The results can provide reference for the structural design of the ship.