Abstract: In this paper, the factors to influence the vertical resonance of a simply supported girder bridges are investigated systematically. The vehicle-bridge interaction system model is used in the analysis. The results show that the main factor to cause the vertical resonance phenomenon of the bridges is the train抯 axle weight moving loads; other factors (such as the span, stiffness and damping of the bridge, the length of the vehicles, the mass of the vehicle抯 wheel-set, the spring stiffness and damping of the vehicle's suspension system, and so on ) also have important influence on the occurrence and development of the vertical resonance of railway bridge.