In this study, compared with the traditional train-track-subgrade integral coupling three-dimensional finite element model, an optimized method for handling train loads has been proposed. The train-track vertical coupling model was established based on the multi-body system dynamics theory, and the wheel-rail excitation load under the condition of random track irregularity was obtained through numerical calculation. Then the wheel-rail load was imported into the ballastless track-subgrade-natural foundation soil nonlinear numerical analysis three-dimensional finite element model by using the secondary development subroutine. On this basis, the dynamic stress distribution law of subgrade under high-speed moving load was studied and analyzed. The research results show that the vehicle load processing method used in this paper replaces the vehicle-irregular track-subgrade-foundation integrated vibration model under the premise of ensuring the calculation accuracy, reducing the modeling and calculation time cost, which has certain reference significance; The vertical dynamic stress distribution law along the transverse direction shows that the value is larger in the track structure, and the value in the subgrade bed is much smaller than that in the track structure. The surface layer of the subgrade bed and the bottom surface of the subgrade bed appears "saddle-shaped" distribution; The vertical dynamic stress distribution law along the vertical shows that as the depth increases, the vertical dynamic stress gradually decreases, and the attenuation rate in the surface layer of the subgrade bed is relatively large, even exceeding 50%; The vertical dynamic stress distribution law along the longitudinal direction shows that the number of stress peaks equal to the number of bogies is produced in each structural layer. The dynamic stress changes of the track and subgrade during train operation can be regarded as repeated loading and unloading processes; The moving speed of the train has an obvious effect on the dynamic response of the subgrade. When the speed increases from 200 km/h to 350 km/h, the dynamic stress amplitude of each structural layer increases by more than 30%.