Abstract: Needle-punched GCL and HDPE geomembrane are widely used in landfill impermeable liner. The internal shear strength of GCL and shear strength of GCL/GM interface are the key factors influencing the slope stability of landfill composite liner. Large-scale simple-shear tests on hydrated needle-punched GCL+GM composite liner are conducted without forcing the shear failure to occur along a pre-determined plane. The development patterns of displacement of GCL/GM interface and internal displacement of GCL are obtained. The peak shear strengths of composite liner when the rough side of GM contacts respectively with the woven geotextile side and the nonwoven geotextile side of GCL are analyzed. The overall shear failure characteristics of hydrated needle-punched GCL+GM composite liner are revealed. The test results show that the large-scale simple-shear tests can correctly and reasonably simulate interactions of GCL and GM. The ultimate failure surface may change with the increase of normal stress, even there may exist a critical state in which both GCL internal surface and GCL/GM interface are the ultimate failure surfaces.