Abstract: Using a large-scale professional apparatus, the 3D monotonic behavior of the interface considering initial shear stress is investigated, which is between a steel plate and gravelly soil widely used in Geo-engineering. The test results show that: 1) the interface compresses at the start of shear application and then dilates when the shear stress is close to shear strength and the tangential displacement is large enough | 2) the non-coaxial behavior of the interface is obviously observed in tangential deformation when the initial shear stress is considered, and the non-coaxial angle decreases as the shear application continues | 3) the displacement relationship in x-direction and y-direction appears as a line, and the angle between the line and initial shear stress has a good relationship with the angle between shear direction versus initial shear stress | 4) the envelope of peak shear strength is nearly circular, revealing that the interface is isotropic in shear strength | 5) the angle between shear direction and initial shear stress impacts a significant influence on the compression, the normal and tangential displacement relationship and the shear stress-strain relationship of the interface.