Water aeration can reduce the cavitation damage of the drainage structure, which is essential to the safe operation of the hydraulic engineering. Numerical simulation has the advantages of low cost and no scaling effect, and it can produce more comprehensive flow field information, so it has become an effective method for studying aerated flow at present. Based on the self-developed NEWTANK numerical model, a mixing-concentration numerical model was developed to study the aerated water flow in a tunnel. The simulation results show that the simulated flow velocity and the air concentration are in good agreement with the experimental data, which verifies the feasibility of this numerical model. Furthermore, a large air circulation was formed in the tunnel and the reverse air velocity increased from 2 m/s to 5 m/s along the water flow direction. Further results show that when the gate opening was constant and the approaching water head gradually increased from 5 m to 15 m, the aeration of the water was further strengthened, resulting in greater bottom air concentration and aeration thickness at the outlet, which increased from 0.006 to 0.060, 0.08 m to 0.12 m, respectively.