Underflow velocity is an important factor affecting the safety of discharge structures. Based on RNG two-equation turbulence model and volume of fluid (VOF) model of the Fluent software, the simulation method of spillway discharge was studied. The flow regime and water depth of the spillway discharge simulation are almost consistent with the prototype observation, verified that the numerical simulation method is rational. With this method, the distribution of the underflow velocity of the spillway discharge was analyzed. The results showed that the position of actual scouring damage is consistent with the position of the maximum underflow velocity and the maximum gradient of the underflow velocity near the bottom of the numerical simulation. It showed that underflow velocity and its gradient should be one of the main reasons which cause the scouring damage of the spillway.