Abstract A two-and-a-half-dimensional finite element method (2.5D FEM) is applied to investigate the dynamic response of an unsaturated ground subjected to moving loads caused by high-speed train. The partial differential equations of unsaturated porous medium in frequency domain are deduced based on the equations of motion and mass conservation of three phases, with consideration of the compressibility of solid grain and pore fluid. Governing equations of unsaturated soil in 2.5D FE form are derived by using the Fourier Transform with respect to the load moving direction. The track structure is simplified as an Euler beam resting on the unsaturated porous half-space and the viscous-elastic artificial boundaries are used to avoid the energy reflection from the boundary. Numerical simulations demonstrate effects of the degree of water saturation and train speed to the ground vibration and the excess pore water pressure. It is concluded that the degree of water saturation has a different influence on the ground displacement and acceleration. The gas phase has varied influence to the ground displacement amplitude at different train speed level at the track center. A very small amount of gas in the saturated ground largely increases the ground acceleration amplitude at a given train speed. Ground displacements attenuate rapidly with almost the same rate for both high and low train speeds near the track center. The maximum amplitude of excess pore water pressure is located at 1.5–2.0 m beneath the ground surface and decreases significantly as the degree of water saturation decreases.