Abstract: In this paper， the relative motion between ship's water plane and wave surface is discussed， where the calculation formulas are obtained， and a corresponding example is presented. Particularly， the slamming process of the ship bow into water is investigated：the slamming pressure on the section is calculated using three different methods， through which the relation between the peak pressure and the 3D shape and navigation speed is studied. Meanwhile， the ship motion in waves is simulated via AQWA， a hydrodynamic software based on 3D-Potential Flow Theory， and the bow slamming simulation is realized by using the Fluid-Structure Interaction（FSI） algorithm in Ansys/Ls-Dyna （note that the motion of the ship bow is specified in advance in order to accomplish the simulation process）. In this slamming simulation， the fluid is modeled with ALE volume meshes， and the structures are modeled with Lagrangian finite elements. The result shows that the change in the liquid level at a specific point， caused by the entry of adjacent profiles，could increase the slamming pressure. Navigation speed also contributes significantly to the rising of slamming pressure. On the contrary， a small longitudinal deadrise angle at the bottom of the bow may help reduce slamming pressure.