Abstract: When an aerator, with a sudden lateral enlargement and a bottom drop, is operated behind a high-head radial gate in a spillway tunnel with mild bottom slope, its air cavity behind the bottom drop is often partially filled with a certain volume of backwater and water wings protruding above the water surface are generated under sidewall deflection. Based on theoretical analysis and physical model tests, this study demonstrated the main reasons for the air cavity backwater and water wings as two backflow sources and one water withdrawal mechanism, and emphasized the importance of a measure to minimize the angle of water nappe impinging on bottom slab at the reattachment of lower nappe surface. By optimization of aerators and comparative analysis, a new shape of the bottom slab with varying local slope is recommended to reduce the backwater and suppress the water wings. The optimized aerator design suggested by this study, its air concentration along the tunnel, and its effective operation under the range of operating water head were verified in-situ in an engineering project. In addition, relationship between the bottom and side cavities and their influence are also discussed in this paper.