The interface of the multi-medium fluid was caught with Level-Set and the interface was dealt with modified Ghost Fluid Method. The coefficients of JWL about detonations were obtained from isentropic expansion data calculated by Kihara-Hikita-Tanaka (KHT) and were programmed. The energy output of underwater explosion for TNT and PETN was calculated. Results show that energy output calculated by the program is consistent with that calculated by explosion similar rules, and the error is less than 10%.