Rainfall-induced landslides are of frequent occurrence in the south of China. One of the most effective methods to stabilize landslides is to lower the groundwater table by installing horizontal drain. A shallow landslide caused by typhoon Morakot happened along Shangyu - Sanmen Expressway in Zhejiang. Horizontal drains were installed and served as an important part of the slope stabilization scheme. The relationship among rainfall intensity, groundwater level, and slope stability was indicated by field monitoring data. Using software GeoStudio, the stability states of the original slope under rainfall are simulated, based on Fredlund’s Unsaturated Soil Strength Theory. After the rain ceases, the draining processes of the original slope and the slope with horizontal drains are compared. It is obvious that the factor of safety recovers rapidly in the slope with drains. The simulated discharge rate of drain and slope surface, and the change of groundwater table also prove the benefits of the drains, which agree well with the field monitoring data.