Abstract: In this paper,combined with the design of free flow tunnel of a certain hydropower station, a kind of U-shape abrupt offset aeration device for preventing cavitation were studied in common and subatmospheric hydraulic models. Characteristics of such aeration device, including the amount of aeration in the bottom and side cavities behind it, the air now in air vent and above the water surface in free level tunnel, the air concentration of nowing water along the tunnel floor as well as its corresponding protective range againstcavitation erosion and the optimization of aeration device, were analysed and discussed.