In order to investigate the damage modes of warships subjected to underwater blast loading closely, two box-like beams were designed basing on similarity criterion. Dynamic response of the beams floating on water subjected to underwater explosion was investigated by changing stand-off and amount of explosive where TNT charge was located closely under the mid-span of the models. In tests, high-speed photography was applied to obtain the whole process how the objects react against blast loading. The experimental results show that explosion bubble pulse will activize beam to bend severely as a whole when the first wet frequency of beam is approximate to the frequency of bubble pulse. If the ratio of stand-off to maximum bubble radius ranges from 1 to 1.5, beam will be damaged into sagging condition with one or two plasticity hinges, and if it ranges from 2 to 4, beam will take on whipping motion. To sum up, explosion bubble pulse play a greater role in making beams damaged as a whole than shock wave.