In this paper, the vibration behavior of a simply supported steel highway bridge under a moving heavy vehicle is investigated. In analysis, the bridge and vehicle are modelled by a simple beam and a constantly moving single-degree-of-freedom vibrating system respectively, then the effects of the vehicle characteristics, mass ratio, random surface roughness and bump etc. are analysed. From the study, it is known that in general case in which the surface condition of the bridge is not considered, the vehicle can be modelled by a moving load of which the vehicle-pavement interactive force is constant, however, the researches on the surface roughness and vehicle characteristics should be conducted in order to analyse vibration behavior of the bridge with bump and random rough surface.