Different　with　single　circuit　transmission　line,　three-phase　conductors　of　each　circuit　on　1000　kV　double　circuit　transmission　line　on　the　same　tower　are　hung　vertically　with　small　phase　spacing　and　shielded　by　cross　arm　imperfectly,　resulting　in　great　mutual　influence　among　three　phases　and　the　electric　field　distribution　characteristic　of　insulator　strings　different　with　single　transmission　line.　The　three-dimension　finite　element　method　was　used　to　simulate　and　calculate　the　electric　field　distribution　characteristic　of　composite　insulators　and　the　grading　rings　on　AC　1000　kV　single　and　double　circuit　transmission　line　tangent　tower.　The　results　show　when　there　are　hanging　the　same　composite　insulator,　conductor　and　hardware　fittings,　the　electric　field　distribution　of　composite　insulators　and　the　grading　rings　on　double　circuit　transmission　line　is　more　seriously　distorted　than　the　single　line.　For　double　circuit　line,　the　maximum　electric　field　intensity　of　composite　insulator　is　about　22.6%　and　grading　ring　15.4%　higher　than　single　circuit　line.　Thus,　the　mutual　influence　among　three　phases　must　be　fully　considered　in　design　and　test　of　double　circuit　transmission　line　on　the　same　tower.