Abstract: The influence of the area contraction ratio of rotary flow generator on the hydraulic characteristics of level rotary flow has been investigated by the hydraulic model test of an internal energy dissipation tunnel . The area contraction ratio has obvious influence on the flow pattern, discharg e, wall pressure and air pressure inside level rotary flow. For the larger area contraction ratio, the flow condition becomes more complicated, the discharge of the tunnel and the change of the negative pressure in the cavity pocket will be reduced, but the cavity diameter of the level rotary cavity flow and the dissip ation ratio of the system will be increased.