Porcelain　insulator　can　provide　electrical　insulation　and　mechanical　fixation.　[t　is　an　important　kind　of　power　devices.　In　this　paper,　a　full-scale　3D　simulation　model　of　strain　tower　was　established.　By　using　finite　element　method,　the　potential　and　electric　field　distribution　of　porcelain　insulators　were　calculated.　The　comparison　of　calculation　results　between　the　two　situations　(　with　and　without　grading　ring)　is　shown.　The　structure　parameters　of　grading　ring　that　affect　the　potential　distribution　such　as　shielding　depth,　ring　diameter　and　tube　diameter　were　compared　and　analyzed.　Combined　with　engineering　practice,　a　reasonable　design　and　placement　for　grading　ring　was　proposed.　It　can　improve　the　voltage　distribution　of　tension　porcelain　insulator　string　and　reduce　the　electric　field　strength　of　grading　ring.　The　conclusion　has　practical　value　for　actual　construction　project.