Based on the project of the mineral berth in Bethioua，Algeria，this paper introduces the design methods of Antifer cube,which is widely used as the armor block of rubble mound breakwater in the Mediterranean area.Aiming at the local instability of Antifer protective block at the junction of vertical breakwater and rabble mound breakwater during the three-dimensional physical model test of the project,through observing the phenomena of model test and analyzing the damage characteristics of similar breakwater,it is concluded that the main reasons leading to the instability of the armor block at the joint are the increase of concentrated wave height at the joint,the strong impact of partial wave breaking and the decrease of friction between the blocks at the joint.The methods of reasonable arrangement of breakwater direction,increasing the quality of protective armor block,reducing slope and laying submerged breakwater are put forward to ensure the stability of protective armor block at connection,which can provide reference for similar engineering design.