The reliability of ship defensive structure subjected to underwater contact explosions is important both in theory and engineering. Because of the complex response of structure under explosions in which the fluid-structure-interaction should be considered, the problem under discussion could not be solved by analytical method. The Monte Carlo combined with FEM method is used to solve this problem in this paper. The samples of basic random variables that are dynamite density, the elastic modulus and ultimate strength of plate material are generated by the random number generate program. Then, with these samples, the simulations of multilayer plate-shell structure subjected to underwater contact explosions are done by LS-DYNA, and the maximum stress of each plate is obtained, which should be validated to obey the normal distribution. Based on the intensity rule, the destroy index is introduced in this paper, then the destroy probability of each plate and system are calculated using the basic theory of reliability.