A　lab-scale　two-stage　UASB-A/O　combined　process　was　used　to　investigate　the　effect　of　free　ammonia(FA)and　free　nitrite　acid　(FNA)　on　the　short-cut　nitrification　of　municipal　landfill　leachate　rich　in　ammonia　nitrogen　(NH4+-N)　on　the　premise　that　the　COD　and　nitrogen　were　removed　simultaneously.　The　denitrification　and　methanogenesis　were　conducted　in　UASB1　simultaneously　for　the　partial　removal　of　TN　and　COD.　The　effluent　COD　of　UASB1　was　further　removed　in　the　UASB2　by　methanogenesis.　Subsequently,　the　denitrification　of　NOx--N　in　the　returned　sludge　using　the　residual　COD　and　the　short-cut　nitrification　was　achieved　in　the　A/O　reactor.　The　experiment　experienced　three　stages　in　104　d:　the　stable　short-cut　nitrification,　the　damaged　short-cut　nitrification,　and　the　recovered　short-cut　nitrification.　The　results　showed　that　when　the　lowest　FA　was　above　3.1　mg·L-1　the　stable　short-cut　nitrification　could　be　achieved　and　NH4+-N　removal　efficiency,　NO2--N　accumulation　ratio　and　TN　removal　efficiency　were　99%,　95%　and　86%,　respectively.　When　the　FA　was　lower　than　0.6　mg·L-1　with　sufficient　alkalinity　and　extended　aeration,　the　short-cut　nitration　was　damaged　rapidly　and　NO2--N　accumulation　ratio　decreased　to　29%　due　to　the　weak　inhibition　of　low　FA　on　NOB.　FNA　was　lower　than　0.011　mg·L-1　at　previous　two　stages,　which　led　to　the　failure　of　inhibition　on　NOB.　While　FNA　was　increased　considerably　in　the　3rd　stage(the　highest　FNA　was　0.414　mg·L-1)　by　decreasing　pH,　the　short-cut　nitrification　was　recovered　soon　and　maintained　and　NO2--N　accumulation　ratio　increased　to　92%　owing　to　the　cooperative　effect　of　FA　and　FNA.　In　brief,　FA　and　FNA　were　important　factors　to　achieve　and　maintain　the　short-cut　nitrification　of　the　municipal　landfill　leachate.　©　All　Rights　Reserved.