In　order　to　study　the　characteristics　of　conducted　common　mode　EMI　of　flyback　transformer,　based　on　the　analysis　of　the　transmission　mechanism　of　conducted　common-mode　noise　in　flyback　power　supply,　a　new　viewpoint　that　transformer　can　be　regarded　as　common-mode　filter　was　proposed.　Through　the　theoretical　analysis　of　the　capacitive　effect　of　transformer,　the　key　factors　affecting　the　common-mode　noise　suppression　ability　of　transformer　were　defined.　By　optimizing　the　structure　of　the　transformer　winding　and　changing　the　capacitance　distribution　parameters　between　the　primary　and　secondary　windings,　the　common-mode　EMI　characteristics　of　the　transformer　were　improved.　Finally,　a　prototype　of　flyback　power　supply　was　tested.　The　experimental　results　show　that　the　common-mode　noise　of　the　circuit　can　be　effectively　suppressed　by　optimizing　the　winding　mode　or　connection　mode　of　the　transformer.　©　2021,　Harbin　University　of　Science　and　Technology　Publication.　All　right　reserved.