In this research, a deep explosion in the reservoir of a concrete gravity dam has simulated by the coupled Euler –Lagrange finite element method. The validity of explosion hydrodynamics pressure has compared with Taylor empirical formula.  The surface sloshing not considered due to deep explosion. The dam and its retained water excited by detonating large explosive charges in deep water upstream from the dam. The damage plastic model of Lee has applied for concrete and damage pattern detection of dam. The dam responses to the underwater explosion were recorded as crest displacement time history. The result of explosion simulation of one ton TNT has compared with actual damage of Koyna dam due to 6.5 Richter earthquakes in 1976. The crack pattern was similar in two cases, but in underwater explosion test, the crack propagation is stopped before reaching to downstream of dam. Evolution of result was shown that controlled explosion test in the reservoir of a dam can be used as a method for study of hydrodynamics effects on dam body safety.