Based on a case of cavitation erosion damage on the sidewalls at the outlet of a radial gate during the operation of a sediment flushing tunnel in a domestic hydraulic project, hydraulic model tests were conducted to study the flow characteristics and wall pressure characteristics in the outlet region of the radial gate under different opening conditions, and the causes of cavitation erosion damage were thoroughly analyzed. The results show that when the radial gate is fully open, the aeration facility operates well and cavitation erosion damage is unlikely to occur. When the radial gate is partially open, the lateral diffusion angle of the outlet flow increases, the length of the side cavity shortens rapidly, and the pressure downstream of the flow impingement zone drops rapidly, resulting in a relatively small cavitation number in this region and the presence of a clear-water zone on the sidewall, which intensifies the risk of cavitation erosion damage. The extent of the clear-water zone is basically consistent with that of the damage zone.