Abstract: Taking the base section of the typical expansive mudstone of Lanzhou-Wulumuqi High-Speed Railway as a research object, the horizontal and vertical seepage tests of the mudstone on the subgrade are carried out to study the swelling and permeability characteristics of the mudstone under different seepage directions. The research results show that during the horizontal seepage tests and the vertical seepage tests, the volume water content of the swelling soil samples at different locations all have three stages: rapid growth stage in the early period, medium-term convex arc type slow growth stage in the middle period, and basic stable stage in the later period. When horizontal seepage occurs in the mudstone, the mudstone both expands and softens, and the closer the mudstone is to the water injection pipe, the larger the swelling of the soil sample is. The soil sample farther away from the water injection pipe releases the expansion energy in the direction of the softened mudstone, and as a result, the amount of expansion becomes smaller in its vertical direction. The factors affecting the penetration of water into the soil sample mainly include the compaction effect of the upper soil sample on the lower soil sample, the effect of the matrix suction and the effect of gravity. When the vertical seepage occurs in the mudstone from top to down, the final volumetric water content of the soil sample decreases from top to down. The expansion of mudstone will lead to the decrease of permeability, and the expansion of mudstone will undergo the stages of rapid expansion, slow expansion and stable expansion. The research results can provide a reference basis for the construction of high-speed railway subgrade in the expansive mudstone area, and can provide technical reference for the construction of similar areas.