Based on compressive fluid dynamics, we combine Discontinuous Galerkin (abbreviated as DG) method with Level Set (abbreviated as LS) method and Direct Ghost Fluid (abbreviated as DGF) method to study load characteristics of underwater explosion. Firstly, the detonation models of underwater explosion for the charges with different shapes are established to investigate the influence on the shock wave loading and the initial shapes of the bubble. Secondly, the numerical results are compared with the experimental data to verify the validity of LS-DGF-DG method. Finally, ABAQUS is utilized to simulate the deformation of hull grillage subjected to underwater explosion, based on the results of LS-DGF-DG method. In addition, the damage characteristics of hull grillage are investigated through the comparison between the numerical and experimental results. The results show that the cylindrical charge gradually develops into ellipsoidal detonation product and eventually becomes a spherical bubble. Besides, the pressure peak in the radial direction is higher than that in the axial direction, and the pressure peak of the cylindrical charge is higher than that of spherical charge. In addition, the numerical results show good agreement with the experimental ones. It was found that hull grillage is damaged by underwater explosion along with the generation of the crevasse and plastic deformation. Besides, the stiffeners have a great influence on the damage modes of the hull grillage.