In order to study the dynamic responses of stiffened plate under underwater contact explosions, the FEM code LS-DYNA is used to discuss the problem, six different stiffened plates are included. The stiffened plate’s distortion, the size of crevasses in the numerical simulation are analyzed. The position where the maximum plastic strain appears, the effective stress and acceleration are also described. It is revealed that the deformation of stiffened plate is different with the position of the stiffener, but the stiffener can harmonize and reduce the deformation of plate, and the whole structure will be more safety when it is subjected to explosions. So the research of this paper can be help to the design of steel structure explode resistance.