A nonlinear numerical analysis model for three -dimensional finite element of CRTS Ⅲ slab ballast less track -subgrade -natural foundation soil is established, and the wheel/rail load is calculated based on the random irregularity of the track. The wheel/rail load is imported into the finite element model by secondary development subroutine of finite element software, and the equivalent three-dimensional uniform viscoelastic artificial boundary is used to simulate the actual semi-infinite space at both ends of the subgrade around the natural foundation soil. Accordingly, the dynamic displacement distribution of subgrade under high speed moving load is studied and analyzed. The results show that the vertical dynamic displacement of the subgrade calculated by the finite element model is far less than the current standard control value of China’s high-speed railway of 3.5 mm,which meets the requirements. The train model adopts two bullet cars, and the dynamic displacement generatedby the movement of two sets of wheel sets in the middle is superimposed. The peak value of the vertical dynamic displacement in the time history curve of each structure layer corresponds to the number of bogies. In the range of concrete base, the vertical dynamic displacement amplitude of each structural layer of subgrade changes little along the transverse distribution, and the maximum transverse difference of each structural layer is only 0.129 mm. Outside the range of concrete base, the vertical dynamic displacement amplitude of each structural layer of subgrade has a great difference in transverse distribution, and the maximum transverse difference is more than 0.5 mm. With the increase of depth, the vertical dynamic displacement in the subgrade gradually attenuates, and the maximum value is located at the top surface of the foundation bed, which attenuates approximately linearly. The impact of roadbed surface stiffness changes on the system dynamic response is limited. The increase of the roadbed foundation stiffness has a significant effect on reducing the dynamic displacement of the subgrade, which is conducive to the smooth, comfortable and safe running of the train. The increase of the train speed increases the deformation displacement of the subgrade. The change of the foundation soil stiffness has the most significant effect on reducing the dynamic displacement of the system in the subgrade.