The aerodynamic problems caused by high-speed train passing through tunnel become more and more serious with the increase of train speed. In view of the existing line reconstruction which is limited by the terrain and can not be equipped with additional buffer structure in tunnel, we proposes a new type of inter-stepwise buffer structure. In this paper, the flow field between train and tunnel was modeled according to the k-ε two equation turbulence model, the information exchange was realized by sliding grid technology. The pressure oscillation was calculated for the initial compression wave generated when a train enters the tunnel with buffer structure at high speed. The simulation results illuminate that the inter-stepwise buffer structure has limited effect on reducing pressure peak, but it can delay pressure rise and greatly reduce pressure gradient peak. When the buffer structure is located 60 m away from tunnel inlet, the expansion form is three-way uniform expansion, the order is first, the length is 30 m, and the cross-sectional area is 150 m2, the mitigation effect of the inter-stepwise buffer structure on tunnel aerodynamic effect is the best, and the pressure gradient mitigation effect reaches 24.66%.