The mechanisms of cavitation damage in flood releasing tunnels remain unclear. In this study a series of physical experiments and numerical calculations were conducted to investigate the flow pattern, pressure distribution and cavitation downstream of a sudden fall-expansion aerator in a discharge tunnel. When the radial gate was partly open, the bottom cavity length reduced drastically, the lateral cavity disappeared, and the flow cavitation index near the sidewalls was less than 0.2. The pressure on the floor and sidewalls can be divided into four regimes: the cavity regime, the impact regime, the reflective regime and the stable regime. The time-average pressure is subject to a unimodal distribution when the gate is fully open, whereas a bimodal distribution is presented when the gate is partly open. The negative pressure regime presented an elliptic shape. Cavitation erosion occurred easily on lateral expansion sidewalls in the tunnel with the radial gate partly open.