Abstract: Hydraulic characteristics of sidewall aerators in high-head large-discharge spillway tunnel have been studied by adopting a model test approach in this work. We made an analysis on the factors of aeration effect and the causes of sidewall water-wings and obtained two shapes of water wing-free sidewall aerators. These new shapes have good air-entraining behaviors and an ability to inhibit the formation of water wings on the sidewall via controlling the length of side cavity. The results also show that generally sidewall aerators produce a certain effect on the air entrainment near the bottom, and thus the bottom aerator and side aerator should be optimized as a whole. Finally, a discussion is presented on design principles and optimization methods for sidewall aerator shapes, based on a comprehensive analysis on the hydraulic characteristics of sidewall aeration.