Abstract: The final anomalous sag distortion of ship girder subjected to near field underwater explosion (undex) below the middle ship was researched. Firstly, the sinking exercises of Spruance class destroyer DD973 sunk by one MK48 torpedo was presented and the simulation model was established. The exponential attenuation phase, reciprocal attenuation phase, post reciprocal attenuation phase and bubble pulse phase of undex load were precisely applied on this model. Fluid-solid interaction, added water mass, gravity and the change of buoyancy were also taken into account in this model. After that, the similarity theory was adopted to analyze the dynamic response of this ship girder. The similarity parameter and theory prediction formulae of the dynamic response of this ship girder were presented. Finally, the physical meaning and the influence of these similarity parameters were analyzed.