Overseas projects in the sea area often face a main direction swell with a period of over 10 s.The Ashdod breakwater in Israel faces the open Mediteranean sea and is affected by long period swell.Based on studies on the physical model tests of breakwaters,we analyze the influence of swell on the stability of breakwater combining with the drain rate of the core rock,1～3 t and 3～6 t armor rock of breakwater under the action of impacts and erosions of waves with long periods and big heights during the construction of the breakwater.We also get the critical instability wave hight of the core rock,1～3 t and 3～6 t armor rock to find out the best installation opportunity.The result may serve as reference for similar projects.