Shear characteristics of the interface between sand and the suction caisson wall play animportant role in determining the friction resistance along the suction caisson wall. In thisstudy, a variable normal load (VNL) reflecting the effects of the penetration depth andinstallation rate of a suction caisson is applied using the torsional interface shear apparatus toexamine the interface shear behavior. Results show that the interface friction coefficientdecreases with the increase of the normal stress and shearing rate until reaching a criticalvalue. When the friction coefficient reaches the critical value, the friction coefficient changeslittle with the increase of normal stress. The variation of the friction coefficient caused by theseepage flow and penetration rate should be considered for calculating the frictional forcealong the caisson wall. Given the same shearing rate, the normalized efficiency parameter ofthe interface under the VNL is higher than that obtained under the CNL. The suctioncalculated based on the test results of this study is in good agreement with that from the Tenbyproject, indicating that our findings are reliable and beneficial to analyze the installationprocess of the suction caisson.