Abstract: A simplified theoretical model to predict the dynamic response of a warship's hull girder subjected to underwater explosion is investigated and presented. In the model, the hull girder is simplified as a uniform ship-like beam and TNT charge is located under the mid-span of the beam. The pressure curve of explosion load is divided into five stages, and the pressure distribution functions are deduced from the wall-pressure characteristics of shock waves and following bubble oscillations. The characteristics of the beam's pure elastic and elastic-plastic motion are analyzed, including the process of loading and unloading repeatedly in plastic deformation, and the influence of variable parameters, such as stand-off and beam's length. Finally, a ship-like beam was used in the experiment to verify the method. The results show that the experimental data basically agrees with those of theoretical analysis. The theoretical method can reasonably capture the essential motion features, such as response cycle and deformation amplitude. When the underwater non-contact explosion occurs closely below the hull girder's center, and the first pulse frequency of the bubble is equivalent to the first-order natural wet frequency of the girder, sagging damage will occur easily.