Abstract: According to the working condition of high-speed railway, a spatial time-varying model of subgrade/track/vehicle coupling system is established for dynamic analysis of subgrade. In the spatial model, the railway structure and vehicle are dispersed into different elements, the dynamic equation of the coupling system is founded with the energy theory, and the vibration equation made up of the coupled matrixes of the system is also established. Under the continued harmonic irregularity, dynamic analyses are performed and used as the verification of the model. The dynamic responses such as the acceleration of the car, the dynamic wheel load, the subgrade dynamic stresses, the subgrade dynamic deformation,etc., are analyzed during the running of a vehicle. The results show that the calculation model presented in this paper is rational.