Because of density difference between saltwater and freshwater is formed a transition zone between two fluids in coastal aquifers. Forward rate and extent of saltwater transition zone depends on several factors including: changes in sea level, aquifer characteristics, hydrological conditions of upstream, tidal and seasonal fluctuations of seawater. One of ways for prevention of seawater intrusion in coastal aquifer is construction of underground dam. In this paper, changes of hydraulic gradient in groundwater aquifer by construction of underground dam is caused to reduce seawater intrusion to coast. The multiphase flow is simulated by computational fluid dynamics method.