Abstract: The interface strength between the suction bucket and the clay is a critical parameter in evaluating the penetration resistance during installation. Since the seabed is completely submerged, factors such as saturation degree, drainage conditions, and pore water pressure significantly influence the development of interface strength. Current interface testing devices do not provide accurate measurement and control of interface saturation, drainage status, and pore water pressure, which limits their ability to reflect the soil-structure interface characteristics in ocean engineering. By modifying traditional strain-controlled triaxial instruments, this study achieved fully undrained interface shear and accurate measurement of pore pressure in saturated clay. Interface shear was evaluated at three shear rates, three roughness levels, and four consolidation pressures. During the tests, interfacial shear stress, deviatoric stress, pore water pressure, friction angle, and stress path were analyzed. The results indicate that as the shear rate increases, both the effective stress ratio and effective stress friction angle increase. In addition, the maximum pore water pressure increases with increasing structural surface roughness. The interface shear characteristics are primarily influenced by the rate effect. As shear rate increases, the axial strain required to stabilize the interface pore water pressure also increases. Hence, careful control of clay saturation and undrained conditions are essential for accurate measurement of interface shear strength, which is crucial for reliable calculation of frictional resistance and suction pressure during suction foundation installation.