Abstract: A dynamic model of a train-ballast track-subgrade coupled system is established. In the model, spatial and time varying characteristics and interactions between the rolling stock, ballast track and subgrade are fully considered. The subgrade are modeled based on continuum theory and then be discretized by utilizing one of Galerkin methods. The model is reliable by comparing the calculation results with the test results in Qin-Shen passenger line. When the CRH2 EMUs passing at a speed of 200km/h, the dynamic characteristics of subgrade are studied and analyzed. The dynamic responses such as the deformation and stress of subgrade surface, the dynamic deformation and stress distribution along the longitudinal and lateral directions are obtained. Such a model performs a fast calculation for the train-ballast track-subgrade coupled system, which can be used to study dynamic interactions between a train, ballast track and subgrade and make a detailed analysis about the dynamic characteristics of subgrade.