The commerical finite element software MSC.Dytran was used to numerically investigate the damage modes of stiffened plates under underwater explosion loads in the case of the different charge quantities and stand-off distances. Some experiments were conducted to confirm the numerical results. The factors which affected the damage mode of stiffened plates were analyzed. On the basis of the analyzed results the critical loads were proposed to distinguish the different damage mode. At the same time the criterion between the contact explosion and the noncontact explosion was put forward from the damage modes of stiffened plates.