The invention discloses an impermeable gasket for a bottom side slope of a refuse landfill. A underground water drainage layer of a geotechnical composite drainage net, a GCL auxiliary impermeable layer, a high-density polyethylene geotechnical film main impermeable layer, a drainage layer of the geotechnical composite drainage net, a geotechnical grid reinforcing rib layer and a bagged soil protecting layer are sequentially arranged from bottom to top, wherein a smoother surface formed by the bagged soil and the geotechnical grid effectively reduces the shearing force transmitted to a lower gasket system in the refuse settling process; the geotechnical grid reinforcing rib layer bears the pull forces of the geotechnical film and the GCL, effectively reduces the adverse effect of the refuse settling and improves the safe reliability and the durability of the impermeable gasket of the bottom side slope. The invention has good impermeable effect and good refuse settling resisting capability, which can better achieve the impermeable function on the side slope of a valley type landfill side slope.