ABS　T　R　A　C　T　In　mature　landfill　leachate　with　high　ammonia　nitrogen　and　extremely　low　biodegradable　organic　matter,　it　is　difficult　to　achieve　advanced　nitrogen　removal　through　traditional　nitrification-denitrification　processes.　In　this　study,　a　low-carbon/nitrogen　mature　landfill　leachate　treatment　process　with　an　anaerobic/oxic/anoxic　strategy　was　constructed　by　inputting　cofermentation　liquid　of　corn　straw　and　excess　sludge.　Under　the　effect　of　the　co-fermentation　liquid,　partial　nitrification　was　enhanced.　Meanwhile,　with　the　cooperation　of　the　heterotrophic　nitrification-aerobic　denitrification　process,　the　nitrite　accumulation　ratio　and　simultaneous　nitrification　and　denitrification　ratio　reached　99.07　+/-　0.52%　and　23.26　+/-　1.02%,　respectively.　The　carbon　source　from　the　cofermentation　liquid　was　efficiently　converted　into　polyhydroxyalkanoates,　which　could　drive　endogenous　denitrification　to　achieve　advanced　nitrogen　removal.　The　nitrogen　removal　efficiency　in　this　system　reached　98.53　+/-　1.21%.　In　addition　to　effectively　supplying　carbon　sources,　the　co-fermentation　reactor　also　achieved　a　sludge　reduction　ratio　of　24.78　+/-　1.68%.　Through　metagenomic　analysis,　the　cooperative　mechanism　by　which　denitrifying　glycogen-accumulating　organisms,　heterotrophic　nitrifying-aerobic　denitrifying　bacteria　and　ammonia-oxidizing　bacteria　achieve　nitrogen　removal　and　nitrite　accumulation　in　oxic　stage　was　revealed.　This　study　provides　a　new　and　efficient　treatment　process　for　mature　landfill　leachate　and　sludge　reduction.