Abstract: Investigation of the mechanism of rainfall-induced colluvium landslides and a reliable evaluation method for the colluvium slopes are essentially important for theoretical researches and practical projects. A fully instrumented laboratory model test system for rainfall-induced landslides is developed. Three model tests are conducted for three types of granular soils to investigate the seepage, deformation and particles migration of loose colluvium soil slopes under rainfall conditions, and the effects of particle-size distribution on infiltration and slope stability of colluvium slopes are discussed. The results show that the volumetric water content, pore water pressure and soil suction in the slope vary with rainfall infiltration. When the wetting front reaches the corresponding measuring point, the volumetric water content and pore water pressure continue to increase while the suction continues to decrease with time. After some time, the measured values become stable. Once the rain stops, the pore water pressure and water content response immediately and gradually decrease, while the soil suction in the slope gradually increases. The displacement of the slope is accelerated when a failure occurs. The initial particle-size distribution, e.g., the content of stone, has a significant impact on the failure modes. The failure modes for the three slopes, which are composed of colluvium soils with stone content of 13%, 19% and 41%, respectively, are multi-level retrogressive sliding failure, shallow sliding failure, massive sliding failure, correspondingly. With a smaller stone content, the slip surface is deeper. The transport of fine particles is more remarkable and the content of fine particles near the toe of a slope is greater when the stone content increases.