In order to study the soil-water interaction mechanism and the cause of landslide of a swelling rock slope under precipitation infiltration, an artificial rainfall test was conducted on two bare swelling rock slopes with different gradients in the Luwangfen test section of the Middle-route of the S-N Water Transfer Project, the responses of infiltration intensity, water content, suction and horizontal displacement of the slope to the rainfall were monitored. The study from the monitored data proves as follows ∶① The gradient of slope and the rain intensity have no influence to the infiltration intensity when the water supply intensity exceeds the infiltration ability of slope. ② In the slope, the soil stratum is affected most severely by rainfall inside the depth of fissures, where the water content fluctuates widely in the course of wetting and drying cycles. ③ The matric suction of the superficial layer will be greatly decreased owing to the existence of backwater zone. ④ The invasion of water is only one inducement of the failure of a gentle swelling rock slope, while the development of fissures and the weakening of the shearing strength caused by wetting and drying cycles is the essential cause.