Anaerobic　ammonium　oxidation　coupled　with　partial　nitritation　is　critical　for　cleaner　production　in　sewage　treatment.　The　long-term　effects　of　high-　and　low-strength　influent　ammonium　(NH4+-N)　on　the　anammox　activity,　ecological　niche　characteristics　and　active　microbial　community　were　investigated　in　a　one-stage　partial　nitritation/anammox　(PN/A)　process.　The　total　nitrogen　(TN)　removal　efficiency　was　up　to　90%　with　influent　NH4+-N　of　192　mg/L.　The　N-15-isotope　pairing　technique　illustrated　that　the　potential　anammox　rate　could　reach　to　3507.8　nmoL/g-sludge/h,　accounting　for　73.2%　of　dinitrogen　production.　As　the　influent　NH4+-N　decreased　to　63　mg/L,　the　anammox　population　significantly　decreased　and　the　Nitrospira　became　the　dominant　specialized　species　in　the　PN/A　system.　The　Nitrobacter　had　the　smallest　niche　overlap　value　and　the　furthest　ecological　distance　to　the　anammox　bacteria　among　the　seven　investigated　nitrogen　conversion-related　genes　along　the　influent　NH4+-N　concentration　gradient,　indicating　different　ecological　similarities.　The　redundancy　analysis　showed　that　the　rise　of　dissolved　oxygen　caused　by　low　NH4+-N　might　be　the　main　cause　of　the　excessive　proliferation　of　the　Nitrospira.　The　N-15-DNA-stable　isotope　probing　illustrated　that　both　the　class　Anaerolineae　and　Proteobacteria　had　closely　symbiotic　relations　with　the　Planctomycetacia　in　this　in　situ　surveys.　This　study　provides　a　deep　understanding　of　PN/A　process　treating　low-ammonium　mainstream　wastewater　from　the　viewpoint　of　microecology.