A　combined　system　consisting　of　an　up-flow　anaerobic　sludge　blanket(UASB)　and　anoxic/oxic　bioreactor　was　adopted　for　treating　mature　municipal　landfill　leachate　rich　in　ammonia　nitrogen(NH4+-N).　On　the　premise　of　the　COD　and　nitrogen　being　removed　simultaneously,　the　sludge　distribution　and　DO　concentration　in　UASB　and　A/O,　denitrification　and　methanogenic　activity　in　UASB,　nitrite　accumulation　ratio　variation　in　A/O,　organic　matter　and　nitrogen　transition　behavior　in　the　combined　system　were　analyzed　at　different　recycling　ratios(100%-600%).　The　results　with　the　recycling　ratio　maintained　at　an　appropriate　level　indicated　that　the　sludge　distribution　characteristics　and　DO　concentration　in　respective　reactors　were　in　a　relatively　better　state.　Gas　production　analysis　showed　that　denitrifying　bacteria　and　methanogen　activity　in　UASB　were　maintained　at　homeostasis　balance.　Under　the　premise　of　ensuring　the　efficiency　of　nitrification,　the　selective　inhibition　of　FA　on　ammonia　oxidizing　bacteria　and　nitrite　oxidizing　bacteria　realizing　control　on　nitrification　rate　to　achieve　a　high　rate　of　nitrite　accumulation　in　A/O.　According　to　the　characteristics　of　liquid　water　and　considering　energy　consumption,　an　optimal　recycling　ratio　of　300%　was　suggested　to　achieve　optimum　system　performance　with　the　removal　efficiencies　of　NH4+-N,　TN,　COD　reaching　99.3%,　85.9%　and　90.6%　respectively　and　NO2--N　accumulation　ratio　reaching　94.4%.　©　All　Rights　Reserved.