Abstract: There is wide distribution of silty clay layers with weak expansion in piedmont area of western Yunnan in China. The rainfall-induced landslides are so frequent that huge losses occur in relevant traffic routes and construction facilities. It becomes one of the most typical landslide hazard in the area. Based on the study of the characteristics of the unsaturated soil, important progress has been made. The soil-water characteristics between matrix suction and water content was proposed by the soil suction test. The saturated water content is about 37%, the residual water content about 3.5%, the air entry value about 190 kPa. The suction internal friction angle 4.3° of the unsaturated clay was achieved by the triaxial mechanical test. The saturated-unsaturated seepage numerical model has been established to simulated the courses and mechanisms of this kind of landslide instability under the conditions of different rainfall intensities. The time effect of the instability during the heavy rain is about 2 day when the landslide stability is significantly reduced. The landslide may become unstable and slide after three days of the continuous heavy rain at 16 mm/day. There is no significant impact on the landslide stability at the rain less than 6 mm/day. The results can be taken as the important scientific basis for prevention of this type of landslide disaster.