The problem of micro-pressure waves caused by the high-speed train passing through the tunnel from the open line become more and more serious with the increase of the train speed. The combined buffer structure can better mitigate the impact of micro-pressure waves than the single type of buffer structure. In order to further alleviate the impact of micro-pressure waves on the surrounding environment of the tunnel, a buffer structure named ladder ventilation type combined buffer structure was proposed. Based on the standard κ-ε two-equation turbulence model (κ is the turbulent kinetic energy, ε is dissipation rate) and the numerical simulation method, four ventilation window parameters were studied, including the ventilation distance, the ventilation ratio, the number of ventilation windows, and the position of ventilation windows. The initial compression wave pressure and pressure gradient caused by the train entering the tunnel with different ventilation windows were analyzed, and the optimum parameters of the combined buffer structure were obtained. The buffer structure has less impact on the maximum pressure of the initial compression wave, but more impact on the initial compression wave pressure gradient. When the ventilation distance is 4 m, the ventilation rate α is 36%, the number of ventilation windows is 2, and the position of ventilation windows is the top ventilation, it is the optimal working condition, and the mitigation effect on pressure gradient reaches 60.14%.