The subgrade mud pumping of heavy-haul railway is widespread and harmful, which seriously affects the stability of tracks and the safety of train operation. The properties of railway subgrade soil, such as the particle gradation, the void ratio, have a significant impact on the characteristics of subgrade mud pumping under train load. In this study, a series of remolded samples consisting of silty clay and different content of Kaolin was subjected to mud pumping tests with a self-developed test model. The influence of varying particle gradations (Kaolin content) and initial dry densities (void ratio) on axial strain, excess pore water pressure, and fine particles migration under dynamic load were analyzed. The results show that with the increase in Kaolin content and initial dry density, the axial strain and the excess pore water pressure of the samples, the average migration height of fine particles, and the degree of subgrade mud pumping all decrease. We also find that the excess pore water pressure gradient is the leading factor driving the migration of fine particles in subgrade soil, and the interlayer has a promoting effect on the occurrence of subgrade mud pumping.