Abstract: A catastrophic landslide of construction waste occurred at the Hong’aocun Landfill in Shenzhen of Guangdong Province at 11:40 am on December 20, 2015, with an area of 38.5×104 m2 covered by soil. Analyses of previous researches on the characteristics and mechanism of the landslide and data of field investigation indicate that the reason of the hazard is that the weak sliding interlayers, which were formed by the saturated clay at the bottom of the landfill, caused the global instability of landslide. In this study, constant permeability test and physical simulation test were conducted to investigate the permeability and the reason of the saturated bottom clay. The experimental results show that the hydraulic conductivity of the infilling material is much higher than the normal clay. What’s more, hydraulic conductivity has an exponential relationship with the original moisture content and dry density of infilling, with the correlation coefficients larger than 0.96 and 0.925, respectively. The hydraulic conductivity of infilling is anisotropic, showing a higher permeability in the vertical direction than in the horizontal direction.