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When a vehicle is driven into a road tunnel, the appearance of tunnel wall influences the flow field around the vehicle and generates additional aerodynamic forces acted on the vehicle, which may alter its road holding and thus result in safety problems. That is one of the reasons why road accidents are more liable to take place in tunnel. In this paper, dynamic mesh technique was adopted to realize the three-dimensional transient numerical simulation of a van running into a road tunnel, and then the aerodynamic forces acting on a van during the process of entering a road tunnel were investigated. The results show that the van's aerodynamic characteristics being in and out of the road tunnel are obviously different. The drag coefficient Cz in road tunnel increases more than 13% compared with out of tunnel, the lift force coefficient Cy also has a considerable increase of 50%, and side force coefficient Cx caused by unsymmetry distances from van side faces to tunnel wall changes slightly. During the process of the van running forwards, aerodynamic forces changes sharply near the entrance of the tunnel.