In　this　study,　a　biological　system　consisting　of　an　up-flow　anaerobic　sludge　blanket　(UASB)　and　anoxic-oxic　(A/O)　reactor　was　established　for　the　advanced　treatment　of　high　ammonium　urban　landfill　leachate.　The　inhibitory　effect　of　free　ammonia　(FA)　and　free　nitrous　acid　(FNA)　on　the　nitrifying　bacterial　activity　was　used　to　achieve　stable　nitritation　in　the　A/O　reactor.　The　results　demonstrated　that　the　biological　system　achieved　chemical　oxygen　demand　(COD),　total　nitrogen　(TN)　and　NH4+-N　removal　efficiencies　of　95.3,　84.6　and　99.2%,　respectively　at　a　low　carbon-to-nitrogen　ratio　of　3:1.　Simultaneous　denitritation　and　methanogenesis　in　the　UASB　could　improve　the　removal　of　COD　and　TN.　Nitritation　with　above　90%　nitrite　accumulation　was　successfully　achieved　in　the　A/O　reactor　by　synergetic　inhibition　of　FA　and　FNA　on　the　activity　of　nitrite　oxidizing　bacteria　(NOB).　Fluorescence　in　situ　hybridization　(FISH)　analysis　showed　that　ammonia　oxidizing　bacteria　(AOB)　was　dominant　and　was　considered　to　be　responsible　for　the　satisfactory　nitritation　performance.