A　continuous　plug-flow　multistage　anoxic/oxic　(A/O)　system　based　on　one-stage　partial　nitrification　coupled　anammox　(PNA)　process　with　integrated　fixed-film　activated　sludge　(IFAS)　was　established　and　operated　over　400　days.　A　step　feed　strategy　effectively　controlled　free　ammonia　concentration　and　alleviated　impacts　on　ammonia　oxidizing　bacteria　(AOB)　and　anammox　bacteria　(AnAOB).　During　day　301-405,　98.1%　of　total　inorganic　nitrogen　was　removed　from　mature　landfill　leachate,　whereas　chemical　oxygen　demand　(COD)　removal　efficiency　was　52.9%.　With　the　enrichment　of　AnAOB　in　oxic　biofilm,　nitrogen　removal　via　the　anammox　pathway　reached　94.3%-95.0%.　During　system　operation,　the　dominant　anammox　genus　shifted　from　Candidatus_Brocadia　to　Candidatus_Kuenenia.　Fluorescent　in　situ　hybridization　(FISH)　indicated　AnAOB　encapsulated　by　AOB　colonies　were　mainly　distributed　inside　of　the　biofilm,　which　promoted　nitrite　utilization　by　the　anammox　process.　This　innovative　system　and　the　results　are　of　great　value　to　practical　applications.