Abstract:Rock fragments in purple soil can directly affect soil water diffusion. The past research mainly focused on the rock fragments with particle size > 2 mm, while the particle size of < 2 mm was ignored. Therefore, based on discussing the influence of the particle size > 2 mm on the water migration of purple soils, it is of great significance to clarify the influence of < 2 mm rock fragments on the moisture diffusion of purple soil containing rock fragments, thus providing theoretical basis for the construction of soil hydrodynamic model. Purple shale and mudstone in Sichuan basin and their developed soils (dark brown purple soil and red brown purple soil) were taken as research objects, three kinds of rock fragments particle sizes (0.25~2, 2~5, 5~10 mm) and four kinds of rock fragments contents (0, 30%, 50%,70%) were set. The horizontal soil column suction method was used to determine the progress of wet front of the soils containing rock fragments, the water diffusivity D(θ) and the soil moisture content θ, Fitting the relationship between D(θ), Boltzmann parameters λ and θ. The result showed that for dark brown purple soil, when the particle size of the rock fragment was 2~5 mm, with the increase of rock fragments contents, the advancing rate of soil wetting front and D(θ) both had a tendency of increasing first, then decreased and finally increased; while when the particle size of the rock fragment was 0.25~2, 5~10 mm, with the increase of rock fragments contents, the advancing rate of soil wetting front and D(θ) gradually increased. For red brown purple soil, when the size of rock fragment was 2~5, 5~10 mm, the D(θ) of 70% rock fragments content was obviously larger than that of the other three rock fragments content of soils. The D(θ) of 30% rock fragments contents and 50% rock fragments contents were not significantly different. The advancing rate of soil wetting front had a tendency of increasing first, then decreased and finally increased. While when the particle size of the rock fragments was 0.25~2 mm, with the increase of rock fragments contents, the advancing rate of soil wetting front and D(θ) gradually increased. Therefore, with the increase of rock fragments contents, the soil water migration rate showed an increasing trend as a whole, and the water migration rate of soil which was developed by mudstone was higher than that by shale.