Abstract: The internal face of GCLs is one of the weak interfaces within the composite impermeable liners. Their internal shear strength is enhanced by the reinforced needle-punched fiber. Through comprehensive analysis of the existing internal shear test results of needle-punched GCLs and large simple shear test results of GCL+GM composite liners, small peak stress is found to appear at small displacements in internal shear stress-displacement curves of hydrated GCLs. By conducting shear tests on hydrated GCLs-encased sodium bentonite, combined with theoretical analysis, the occurrence of the small peak stress at small displacements is confirmed to be the inherent feature in the stress-displacement curve of hydrated GCLs, and the small peak stress represents the shear strength contribution from GCL-encased bentonite. On the basis of the small peak stress phenomenon in the stress-displacement curves of needle-punched GCLs, the contribution of the reinforced fiber to the shear strength is obtained quantitatively, and the whole process of internal shear failure and stress-displacement development of GCLs is analyzed based on the failure mechanism. Considering the contribution of each part of needle-punched GCLs to the shear strength, a peak shear strength criterion model which can reflect the failure mechanism of needle-punched GCLs is proposed.