With　the　development　of　communication　technology　and　the　rapid　augment　in　data　and　multimedia　services,　the　demand　for　indoor　localization　is　increasing.　Radio　Frequency　Identification　(　RFID)　based　real　time　localization　system　is　an　effective　positioning　system.　However,　due　to　the　complex　multi-path　interference　of　indoor　environment,　most　of　the　localization　methods　are　lack　of　positioning　accuracy.　In　this　paper,　we　present　a　multi-frequency　carrier　phase　difference　based　ranging　measurement　for　passive　RFID　system　in　the　real　experiment　scenario.　Kalman　filter　is　used　to　calibrate　the　phase　difference　value　and　a　least　square　method　is　employed　to　estimate　the　environmental　influencing　factor.　Real　environment　experiment　results　indicate　high　ranging　accuracy　of　our　approach,　which　lay　a　foundation　for　further　two　or　three　dimensions　measurement.　The　paper　explains　the　basic　measurement　principle,　experimental　setup　and　performance　analysis.