This study presents an experimental investigation on the microstructure and the hydraulic properties of a coarse-grained soil used in high-speed railway track bed at various compaction degrees. A large-scale infiltration column was adopted to study the soil water retention curve (SWRC) and the hydraulic conductivity. The microstructure of the fines in the samples was investigated by scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) tests. The results reveal the existence of a characteristic pore diameter as 12.1 μm, separating two pore groups: macropores (pore diameter≥12.1  μm) and micropores (pore diameter<12.1<µμm). According to the Young-Laplace law, this characteristic pore diameter is consistent with the matric suction (negative pore water pressure, the difference value between the air pressure and the water pressure) value of 24.0 kPa. This value is also the characteristic matric suction ψcha identified from the infiltration column test. When the matric suction is lower than ψcha, the hydraulic properties of this soil are primarily affected by the macropores. As the compaction degree increases, the macropore content decreases, leading to the decreases of the water retention capacity and the hydraulic conductivity in this suction range. However, with the matric suction higher than ψcha, the micropores more significantly affect the hydraulic properties. As the compaction degree increases, the micropore content increases, resulting in the increase of the water retention capacity and the hydraulic conductivity at this suction state.