Vortex-induced vibration (VIV) occurs at low wind speed under normal condition, affected greatly by section type of bridge and its ancillary facilities. Few researches on VIV of vehicle-bridge system are conducted, and many researchers pointed that vehicle-bridge system should be paid attention to in wind tunnel test. In this paper, type of vehicle, number of vehicle and distance of traffic flow’s influence on VIV were studied with a flat steel box girder model. Results obtained from wind tunnel tests were displayed in order to prove the following rules: First of all, compared with conditions without vehicles, response of VIV is reduced severely when a vehicle is placed on the bridge. Secondly, response of VIV and range of VIV with three vehicles are greater than one vehicle on the bridge. At last, VIV is reduced when distance of traffic flow increases. Mechanism from fluid dynamic point view is given by simulating the working conditions with ANSYS 14.5.