It　is　pointed　out　in　this　paper　that　the　p-q,　ip-iqand　d-q　techniques　used　in　active　power　filter　(APF)　design　are　the　methods　based　on　rotor　coordinate　and　the　fan-in　vector　theory　is　the　method　based　on　stator　coordinate.　The　inherent　relationship　of　the　methods　between　stator　and　rotor　coordinate　is　proved　and　the　uniform　expression　is　proposed.　By　comparing　the　superiority　and　applicability　of　algorithms　with　each　other,　an　improved　method　of　fan-in　vector　is　presented　to　solve　the　problem　of　adding　delay　because　of　data　band.　The　method　solves　the　real　time　problem　of　how　to　extract　the　positive　sequence　active　and　reactive　currents　from　an　unsymmetrical　and　distorted　three　phase　system　by　fan-in　theory,　and　simplifies　the　system　structure　and　reduces　the　computation　burden.　Simulation　results　show　its　validity　and　feasibility.