Abstract: The behavior of critical interface in geosynthetic multilayer liner system is an important problem in stability analysis of landfills. The point of single and fixed critical interface is widely adopted. By using the ring simple shear apparatus it is found that the critical interface in geosynthetic multilayer liner system is not single and fixed, and it can shift from one to another with changes of the normal, and two critical interfaces with the same strength may exist in a certain extent of the normal stress. The relationship between shear stress and displacement for the tested geosynthetic multilayer liner system is hardening behavior under various normal stresses. The shear strength of liner system is always lower than that of the critical interface. The test results show that the ring simple shear apparatus can better reveal the shearing behavior for the entire liner system and more reasonably simulate the interaction of various liner materials of a multilayer liner system.