Salt water intrusion in estuaries is very harmful for the estuarine environment. Air curtain system is effectively to prevent it. Experiments were carried out to obtain the relation between the river flow rate and the required gas flow rate under certain discharge of intrusion. The results indicated that, if the bubble plume intensity is strong enough, the salt intrusion is successfully prevented. A two-dimensional numerical model was made by solving the mass and the momentum and the buoyancy conservation equations. The numerical results agree well with the results of the experiments.