<p>Given the significant disparity in material properties between seabed soil and structural foundation, accurate description of the mechanical behavior at soil-structure interface is crucial. This study performed several cyclic interface shear tests to investigate the strength and deformation characteristics at the silty sand-steel interface under multiple boundary conditions. The shear strength, volumetric deformation, mobilized friction angle, and liquefaction trend at the interface were quantified. The increase in fines content transitions the soil skeleton from sand-sand contact to fine-sand, fine-fine contact, shifting the shear behavior from alternating dilation and contraction to dominant contraction. As the number of cycles increases, the differences in both cumulative and cyclic components of normal displacement across successive cycles diminish due to the degradation of interfacial restorability, accompanied by reduction in phase transformation stress. Tighter interaction of soil particles under high-stress condition enhances the liquefaction resistance, while increasing cyclic amplitude allows interfacial dilatancy to be more fully expressed, each contributing to a delayed onset of liquefaction. By introducing characteristic cycles that incorporate the effects of fines content, initial normal stress, and cyclic amplitude, an interface liquefaction formula is formulated to characterize the rapid liquefaction failure at the interface.</p>