Abstract: Supercritical flow over a rectangular splitlitter pier behind which air can be drawn is researched by means of regression analysis and model test at medium and low Froude number.A critical Froude number formula which distinguishes the condition of nonsplashing from that of splashing over is developed by using the momentum equation.Flow splashing over the pier, the relations of demensionless free surface, and that of the drawn coeffcient repectively to the relative pier height, flow contract ratio,and the Froude number are obtained. At last it is proved that the dissipator would never cavitate,provided that air can be drawn behind it.