Abstract: The dosage of geosynthetic clay liner (GCL) is very great in landfills (about 90 percent of its sum dosage).The leachate in the landfills involves a lot of multi-valence cations,such as Ca2+, Mg2+,etc.,Ion exchange between these cations and cations in GCL will influence GTCL'S hydraulic conductivity.This paper presents the condition,mechanism and process of ion exchange between the ions of bentonite in the GCL and ions in leachate and their effects on GCL's impermeability,and some resolution methods are proposed.