Abstract: A series of interface tests between gravel and structure are conducted in stress-controlled two-way circular cyclic shear path by using the large-scale direct-shear apparatus, and the effects of shear stress amplitude on the tangential deformation performances of the interface, including tangential displacement, non-coaxial angle and shear flexibility, are addressed. The test results show that the interface presents distinct 3D behavior subjected to two-way circular cycling of shear stress, such as distinct tangential displacements in the x and y directions and their migration, non-coaxial angle, shear flexibility and shear coupling effect. The tangential displacement amplitude and the peak shear flexibility almost remain invariable with cyclic shearing when the shear stress amplitude reaches the critical stress amplitude. The unity of opposites is discovered between the stabilized non-coaxial angle and the initial peak shear flexibility of the interface. The shear stress amplitude primarily impacts the magnitudes of the performance parameters of the interface, instead of their relationship patterns. Increasing the shear stress amplitude results in magnified tangential displacement amplitudes in the x and y directions, enlarged migration of the tangential displacements towards the negative directions, increased peak shear flexibility and decreased stabilized non-coaxial angle. The stabilized non-coaxial angle and the initial peak shear flexibility have a close relationship with the shear stress amplitude, and can be described using the proposed formulas, which may provide a sound basis for the 3D constitutive modeling of the soil-structure interface.