Xigeda Formation is distributed in southwest of China, and its engineering properties are very poor. In order to analyze seepage characteristics and stability of silty sand slope of Xigeda Formation overlying Quaternary residual slope sediments under heavy rainfall conditions, seepage law of slope is studied by numerical simulation based on Mojiagang landslide in Shimian County; Morgenstern-Price method is used to calculate safety factor of landslide and to analyze their variation laws. The results show that: rainwater infiltrates into slope layer by layer, and seepage process will continue after rainfall. As the existence of soil layer interface, seepage characteristics are different from homogeneous slope; during the rainfall process, the range of pore water pressure growth accelerates near the interface, and the Quaternary residual slope sediments near the interface is still in a near-saturated state after the rainfall; rainfall infiltration leads to reduction of safety factor of the slope, and stability has a lag in response to rainfall.