For　the　optimization　design　of　air　circuit　breaker　(ACB),　it　is　important　and　necessary　to　calculate　the　electro-dynamic　repulsion　force　acting　on　the　movable　contact.　A　method　based　on　3-D　FEM　with　the　equations　that　describe　the　relationships　among　current,　magnetic　field　and　repulsion　force,　which　takes　the　ferromagnet　into　account,　is　adopted　to　calculate　the　electro-dynamic　repulsion　force.　The　method　enables　one　to　analyze　the　factors　that　affect　the　electro-dynamic　repulsion　force,　including　the　number　of　the　movable　conductor　parallel　branches　as　well　as　the　location　of　the　axis　and　the　shape　of　the　flexible　connection.　The　discussion　of　the　calculation　results　is　also　presented　in　this　paper.