Abstract: Influence of ventilation on hydraulic characteristics of level rotary flow has been investigated on the hydraulic model of an internal energy dissipation tunnel. Ventilation has obvious influence on the flow pattern, discharge, wall pressure and air pressure inside level rotary flow. When ventilating, the cavitaty diameter of level rotary flow increases and the flow becomes more stable. With the increase of diameter of ventilation hole, the quantity of both discharge and ventilation first increases then decreases, and reaches a maximum, and the negative air pressure inside level rotary flow quickly reduces and tends to become zero. When not ventilating or the diameter of ventilation hole is smaller, the negative air pressure is becoming larger and the wall pressure also has the behavior, which is diverse from our conventional understanding. With the rise of down- stream water level till the outlet of level tunnel to be just submerged, the quantity of ventilation gets max.