In order to investigate the aerodynamic characteristics of vehicle-bridge system with asymmetric layout of railway lines，a device for measuring aerodynamic force of vehicle-bridge synchronously is introduced. A section model wind tunnel test of a long span cable-stayed bridge with asymmetric layout of railway lines is carried out by the device introduced. The lower-level railway and the lower-level highway are considered as windward side respectively，and the aerodynamic forces on vehicle and bridge under various coupled combinations are measured. The effects of different factors，including asymmetric layout of railway lines，wind attack angle，two trains passing each other and automobile，on the aerodynamic characteristics of the vehicle-bridge system are discussed. The results show that compared with the condition of windward side of the lower railway，the lift coefficient and torque coefficient of the bridge on the windward side of the lower highway are quite different，and the lift coefficient of the vehicle also changes greatly. The drag coefficient of bridge and vehicle decreases with the increase of wind attack angle. When the two trains pass each other，the drag coefficient of the leeward side vehicle changes suddenly. Due to the effect of anti-glare net between rail and road in the same floor，the influence of the automobile on the aerodynamic characteristics of the train is relatively small.