Abstract: For a thorough research on the GCL overlap anti-seepage problem in terms of seepage flux and seepage stability, an overlap seepage physical test method for the simulation of GCL installation process is established by developing an interface seepage experimental apparatus consisting of pressure control system and model test system under normal stress. The optimization of anti-seepage of the overlap bentonite is preliminarily discussed according to the tests under different conditions. The results show that the GCL overlap width should be more than 30 cm. The amount of bentonite added on the overlap should not be less than 4000 g/m², preferably between 4000~5000 g/m². GCL model specification 5000 g/m² is able to meet the requirements of impervious body. The magnitude of confining pressure and the load path both have great impact on the permeability of the overlap. The overlap remains dense and firm and its permeability changes a little if the confining pressure first increases and then decreases.