This　study　established　the　one-stage　partial　nitrification　coupled　anammox　and　partial　denitrification　coupled　anammox　process　in　an　anoxic/oxic　continuous　plug-flow　system　and　operated　for　465　days　to　treat　mature　landfill　leachate.　97.9　%–98.1　%　of　inorganic　nitrogen　was　removed　when　the　nitrogen　loading　rate　was　maintained　at　0.33–0.36　kg　N/m3/d,　and　a　high　anammox　contribution　to　nitrogen　removal　(89.8　%–92.4　%)　was　achieved.　The　long-term　in-situ　free　ammonia　(FA)　anoxic　treatment　contributed　to　the　stable　performances　of　partial　nitrification　and　in-situ　fermentation.　The　employed　integrated　fixed-film　activated　sludge　technology　favored　the　enrichment　of　hzsA,　hzsB,　hdh,　amoA,　hao,　narG,　and　napA　functional　genes.　The　oxic　zone,　particularly　oxic　biofilm,　was　the　key　zone　for　anammox　pathway,　where　Candidatus_Kuenenia　(from　1.6　%　to　8.3　%)　with　high　tolerance　to　FA　and　salinity　stress　outcompeted　Candidatus_Brocadia　(from　18.3　%　to　0.1　%)　as　the　dominant　anammox　bacteria.　This　study　could　provide　guidance　for　anammox-mediated　landfill　leachate　treatment　in　practical　projects.　©　2022　Elsevier　Ltd