The　method　how　to　excite　the　longitudinal　modes　in　the　steel　strands　based　on　magnetostrictive　effect　is　analyzed　theoretically.　The　simulation　model　which　is　a　seven-wire　steel　strand　with　99mm　long,　17.80　mm　nominal　diameter　is　constructed　by　software　Maxwell　which　is　used　for　3D　electromagnetic　field　finite　element　analysis.　According　to　corresponding　theory　of　designing　magnetic　circuit,　double　magnetic　circuits　and　three　magnetic　circuits　are　developed　to　provide　bias　magnetic　for　seven-wire　steel　strands.　Magnetic　circuit　of　surface　and　radial　sections　of　steel　strand　is　finite　element　analyzed.　The　results　show　that　three　magnetic　circuits　have　not　only　stronger　but　also　have　better　uniformity　in　magnetic　field　intensity.