Abstract: Suction caissons are popular for mooring oil and gas platforms and fixing wind turbines. The penetration resistance is mainly the friction between the inner and outer walls during suction penetration in sand. During installation of the suction caisson, the frictional forces between the caisson wall and the soil are dominated by the shear behaviors of the interface between the caisson wall and the soil. A series of interface shear tests with the GDS interface shearing instrument are conducted to investigate the influences of the initial relative densities and penetration depth on the interfacial shear characteristics of soil-suction caisson. The test results show that the shear stress–shear displacement relationship curve of interface under different relative densities exhibits strain softening, and interfacial shear strength and the corresponding shear displacement increase with the increasing relative densities. It is found that the friction coefficient of interface increases with the increase of the relative density, and firstly increases and then decreases with the increase of the penetration depth. Under the experimental programs of tests, the friction coefficient of interface ranges from 0.2 to 0.34, and the friction resistance at different depths can be calculated in terms of the test results, indicating the friction resistance between the inner and outer walls increases exponentially with the increase of the penetration depth. The friction resistance between the inner and outer walls increases with the increase of the relative density, and the increase rate of friction of the outer wall is greater than that of the inner wall. When the penetration completes, the friction of the outer wall is 2.12 times that of the inner wall. During shearing, sand particles are found to be squeezed out, and they are mainly squeezed out at the initial stage of the testing, i.e., the corresponding shear displacement is relatively small. This phenomenon indicates that during installation of the suction caisson, the sand particles are squeezed out because of the replacement between the sand volume and the volume of suction caisson wall. The sand particles broken in the process of testing causes the sand volume to decrease. Besides, it is found that the ratio of the interfacial friction angle to the internal friction angle decreases with the increase of the relative density. To obtain the accurate installation resistance and the required suction to install the suction caisson, it is important to investigate the shear behaviors of the interface between the suction wall and the sand.