Abstract: Firstly, warship’s hull was simplified as a rigid, perfectly plastic uniform beam floating on water, and the theoretical formula of wall-pressure on the wet surface of the beam was obtained. The formula was calibrated by test data of wall-pressure, thus an empirical formula for wall-pressure modification coefficient was presented. Secondly, to analyze dynamic responses of ship-like beam subjected to underwater explosion located closely under it at any stand-off, a theoretical method was provided to describe how the beam acts against blast as a rigid-body movement or plastic deformation. A ship-like beam was designed and experiments were carried out to verify the method’s correctness. Finally, more efforts were focused on the beam’s plastic deformation. The results show that beam’s plastic deformation under shockwave can be looked as a process of plastic rotation companied with a rigid-body movement. Shortening stand-off can increase plastic deformation more efficiently than adding explosives. For the sake of warship’s safety, shortening its length appropriately is more useful in resisting structural distortion than enlarging its width.