The crevasse of clamped square plates under near-field noncontact underwater explosion is studied in this paper. According to conversation of energy, the energy of shock wave translate to the initial kinetic energy of the plate, and then translate to the distortion energy. By means of supposing a certain mode of distortion, the relations between the distortion energy and the angle of the plate are obtained. Therefore, the angle of the plate can be obtained. After the appearance of the damage at the center of the plate, the cracks extend toward to edges, like the leaves, till the ring strain equal to the fracture strain. Then the crevasse get maximum. According to the angle, the size of the crevasse can be obtained. Compared with the results of the FEM software ABAQUS, the theoretical results are believable. So the method of this paper can be used to estimate the crevasse size.