Abstract: A theoretic model for environmental vibrations induced by trains on viaducts is developed in this paper by means of train-track-bridge coupling dynamic theory and finite element methods. The model mainly consists of two parts: the source model and the path model. The source model is made up of a train, the rail and bridge. The train model is a multi-freedom system with car bodies, bogies and wheels connected by spring-dampers. The rail is modeled as Euler-Bernoulli beam, and the bridge is modeled as simplely supported beam with a spring fastening. The path model is established by finite element method software. The theoretic model presented can systematically describe the production and propagation of environmental vibrations induced by trains on the viaduct. The calculated results show a good agreement with the measured results collected on the Beijing No.5 Metro Line in July, 2007.