Using the realizable k-ε turbulence model and the water-gas two-phase flow mixing model, the flow in a curve-floor stepped tunnel with pre-sudden lateral enlargement and bottom drop aerator under different flow rates was numerically simulated, and the distribution rule of aeration concentration in the side wall, bottom plate and flow of the curve-floor stepped tunnel was obtained. The results show that the cavity created by the pre-sudden lateral enlargement and bottom drop aerator is consistent with the test results. The calculated value of the aeration concentration at the measurement point is greater than the test value, but however, the calculated and measured values both increase with the discharge. When the discharge per unit width is greater than 28 m²/s, stable side and bottom cavities through sudden lateral enlargement and bottom drop aerator can be formed. During the flow passing through the step, the aeration effect gradually develops from the surroundings to the inside, and finally a uniform aeration water flow is formed. The joint effect of the sudden lateral enlargement and bottom drop aerator and stairs in the connection section can effectively increase the aeration concentration of the water flow at the bottom and side walls. When the discharge per unit width is 54 m²/s, the aeration concentration of the aeration uniform water flow at the end of the connection section can reach 20%. The provision of pre-sudden lateral enlargement and bottom drop aerator can effectively increase the aeration concentration of the ladder energy dissipator under large discharge per unit width, and can reduce the risk of cavitation damage in flood discharge structures.