In order to study the influence of gas-liquid two-phase interface flow and mixing phenomenon on engineering construction under complex boundary conditions, we combined with the medium spillway of a large hydropower station, used the RNG k-ε model to simulate the three-dimensional flow field, used the finite volume method to discretize the governing equation, and used the GMRES algorithm to solve the pressure problem. Numerical simulation of high speed aerated flow on the discharge wall of the pre-aerated stepped spillway and the traditional stepped spillway was carried out. The results show that the aeration cavity and aeration concentration increased with the increase of the slope of the aerator, and the aeration concentration decreased along the flow, until reaches a certain distance, the concentration reaches the requirement of reducing cavitation damage. Compared with the simulation results of the traditional stepped spillway, the addition of the front aerator can increase the aeration concentration of the first several steps, make the flow reach water and gas balance ahead of time, and do not affect the energy dissipation rate of the stepped spillway. This study provides a new way to solve the engineering problems in the traditional stepped spillway.