To investigate the subgrade mud-pumping of ballastless track for high-speed railway, a full-scale model test system was built. The moisture content, matric suction, and excess pore water pressure of the subgrade during the process of subgrade mud-pumping were measured, the hydrodynamic pressure characteristics of the subgrade were analyzed under dynamic loads induced by high-speed trains, and the mechanism and influencing factors of subgrade mud-pumping of ballastless track were evaluated. Results show that the subgrade surface layer of ballastless track was basically saturated after rainfall infiltration. The mud-pumping was likely to happen under the dynamic loads of high-speed trains. The subgrade mud-pumping occurred with an increase in excess pore water pressure, and the increment of excess pore water pressure was constant in the mud-pumping region with an increase in the depth of subgrade surface layer. In saturated situation, the excess pore water pressure of the subgrade on both sides of the concrete base was higher than that below the center line of the subgrade, making mud-pumping more likely to occur. Reducing the water content or pore water pressure of subgrade can be an effective way to control and prevent the occurrence of subgrade mud-pumping.