The landfill slope is prone to landslides during the rainy season. This problem is one of the key environmental geotechnical problems, existing in landfill at home and abroad. With the development of urbanization, landfill has been a new infrastructure and geotechnical structures. The structure of landfill is very complex. In landfill, the seepage, deformation, stability and diffusion mutual influence and have coupling effect. The permeability coefficient of landfill is an important hydraulic parameter. In this paper, using saturated-unsaturated seepage coupling model; analyzed and calculated permeability coefficient of a landfill in north China, and some useful conclusions are obtained. With the increasing of rainfall time, the safety factor is on the decline. When the rainfall return period is 100 years, the safety factor is the fastest decline after rain 24h. When the rainfall return period is 50 a, 30 a, 20 a, and 10 a, the safety factor declines markedly after rain 15h.