In　this　article,　a　new　kind　of　cascade　connection　method　of　differential　mode　(DM)　active　EMI　filter　(AEF)　is　put　forward.　Contrary　to　the　conventional　idea,　it　is　found　different　kinds　of　cascade　AEF　connections　show　different　insertion　loss　(IL).　Based　upon　the　mathematical　analysis,　among　the　eight　potential　connection　methods,　two　cascade　AEF-connection　methods　are　more　effective　than　the　other　six　methods.　The　IL　is　much　higher　than　the　others.　Therefore,　the　two　effective　topologies　are　further　analyzed　from　several　perspectives.　And　the　best　topology　suiting　the　practical　application　is　found.　Several　prototypes　are　fabricated　and　tested.　Experimental　results　show　good　agreement　with　the　theoretical　calculation.