As a country with frequent earthquakes, the high-speed railway network in China is more and more intensive, and many railways are located along the seismic belt, thus endangering their operation safety. Pile-supported subgrade is widely used in high-speed railways in China. Therefore, the seismic response characteristics of pile-supported subgrade under seismic load were analyzed through a three-dimensional numerical model of track-subgrade-foundation, and the influence of calculation parameters, i.e., pile diameter, pile length, pile spacing, and elastic modulus of pile body on the seismic response of subgrade were discussed and contrasted with those of free subgrade. The seismic response characteristics of free subgrade showed that the vibration displacement amplitude of rail in the horizontal x direction is the largest under seismic load, which is 5 times of that in the z direction; the vibration displacement amplitude at the foot of subgrade slope is 25.2 times of that at the surface of subgrade bed. Compared with the free subgrade, pile-supported subgrade has an increasing effect on the horizontal displacement amplitude and acceleration amplitude of the track, but it reduces the vertical values. It also obviously reduces the displacement amplitude and acceleration amplitude of each structural layer of subgrade. It is concluded that the optimal pile diameter, pile length, pile spacing, and elastic modulus of pile-supported subgrade are 0.8 m, 8-10 m, 2 m and 20 GPa, respectively.