The majority of the overseas port construction projects are located at areas with long period swells.The action of long period swells on the breakwaters is not fully known and researched,especially for the oblique incident waves.Based on one actual breakwater construction project of one port,the 3D physical model tests are performed to study the stability of the rubble-mound breakwater under action of oblique long period swells.Based on the test results,the original plan of the breakwater is optimized,and finally the plan of the breakwater which can satisfy the requirement of stability under wave action is proposed and verified.The test results show that under the action of oblique incident waves with the same water depth,wave directions and wave heights,the damage percentage of the armor rocks of the breakwater is larger for longer wave periods.And under the same water depth,wave heights and wave periods,the impact and erosion actions of the current along the breakwater induced by the oblique incident waves are much stronger under the 15 degree incident wave direction than under the wave direction almost paralleled to the axis of the breakwater,which results in the larger damage percentage of the armor rocks.