Abstract: Needle-punched GCL and HDPE geomembrane (GM) are widely used in impermeable engineering, and the whole shear strength of multi-layer composite liner with GCL is crucial for the slope stability. A large simple-shear testing method for composite liner with needle-punched GCL is introduced, and the whole peak shear strength of composite liner with dry GCL is compared with that with GCL in hydration state. The results show that the shear failure of composite liner does not occur at the dry GCL internal face. The shear strength of composite liner with dry GCL is not always larger than that with hydrated GCL. Combined with the shear failure mechanism of composite liner with GCL, the influence of GCL hydration state is illustrated for the shear strength of composite liner. The translational interface stability of composite liner with GCL in different hydration states should draw engineers’ attention.