The Smoothed Particle Hydrodynamics (SPH) method is used to simulate the dynamics of the underwater explosion (UNDEX) within the steel vessel. Three kinds of simulations, the propagation of underwater shock wave generated by the experimental data, underwater shock wave-metal interaction, and detonation gas-water interaction are performed and are compared with the experiment to provide confidence in numerical modeling. These simulations clarified that the interaction between steel vessel and water is well simulated, but the reproduction of expansions of water and detonation gas are difficult to simulate because the quick increase of volume per one particle cause the oscillation at detonation gas-water interface. As a result, the simulated deformation of steel vessel caused by UNDEX becomes smaller than that of the experiment. The present simulations clarify the advantages and difficulties of SPH method reproducing the damage of explosion including the coupled fluid-structure interaction.