Transformers　are　the　key　components　in　isolated　switching　mode　power　supplies　to　affect　their　electro-magnetic　interference(EMI)characteristics.When　using　previous　three-capacitor　EMI　model　to　evaluate　the　EMI　noise,　the　error　is　greater　than　prediction　when　the　noise　frequency　is　high　or　when　the　transformer　turns　ratio　is　small.In　this　paper,　firstly　the　influence　of　leakage　magnetic　field　on　noise　potential　distribution　was　analyzed,　complemented　with　the　effect　of　the　secondary　side　coupling.Secondly　the　influences　of　the　turn　ratio　structures　on　the　EMI　noise　were　compared　and　summarized.Based　on　these　analyses,　an　improved　EMI　model　of　transformer　with　the　consideration　of　leakage　magnetic　field　was　proposed.The　proposed　model　was　evaluated　by　using　the　parameter　measurement　method,　which　could　describe　the　two-port　characteristics　of　a　transformer.Both　experiment　and　simulation　results　confirm　the　theoretical　analysis　and　model.　©　2017,　The　editorial　office　of　Transaction　of　China　Electrotechnical　Society.　All　right　reserved.