A　system　consisted　of　two-stage　UASB-Anoxic/Oxic　reactor　(A/O)　was　used　to　study　the　short-cut　nitrification　of　the　mature　municipal　landfill　leachate　with　low　COD　and　high　ammonia　concentration.　The　COD　concentration　of　influent　leachate　was　4.3　g/L,　and　the　ammonia　concentration　of　influent　leachate　was　2.8　g/L,　so　the　ratio　of　C/N　was　only　1.5.　The　uasb1　methane　production　and　the　denitrification　happened　at　the　same　time.　The　concentration　of　uasb1　effluent　COD　was　removed　further　in　the　uasb　2.　Then　nitrification　took　place　in　the　A/O　reactor.　The　results　of　the　experiment　showed　as　follows:　First　of　all,　when　we　did　not　add　up　carbon　source　to　the　system,　the　COD　was　used　as　carbon　source　of　denitrification　by　the　first　UASB.　It　was　short　of　carbon　source　of　denitrification　in　the　A/O　reactor.　Then,　when　we　added　up　carbon　source　to　the　first　shelve　of　A/O　system　to　advance　the　denitrification,　because　the　denitrification　can　produce　alkalinity,　which　can　complement　alkalinity　by　nitrification　consumed,　and　maintained　pH　at　a　suitable　scope.　By　this　way,　we　successfully　achieved　an　stable　short-cut　nitrification.　Short-cut　nitrification　with　87%　of　nitrite　accumulation　efficiency　took　place.　But　when　we　did　not　add　up　carbon　source　to　the　system,　the　nitrite　accumulation　of　rate　was　only　20%.　The　ammonia　concentration　of　effluent　leachate　was　10　mg/L.　The　ammonia　removal　efficience　was　99.6%.　But　when　we　did　not　add　up　carbon　source　to　the　system,　the　ammonia　removal　efficience　was　92%.