Although　mainstream　partial　nitrification-anammox　(PN-A)　is　a　highly　efficient　and　sustainable　wastewater　treatment　process,　it　is　difficult　to　achieve　and　stabilize　due　to　the　competition　among　functional　bacteria.　In　this　study,　achieving　one-stage　mainstream　anammox　via　regulating　bacteria　community　structure　was　studied　in　a　lab-scale　biological　aerated　filter　(BAF).　The　results　showed　that　high　free　ammonia　with　89.57　mg/L,　nitrite　nitrogen　(NO2--N)　competition　between　anammox　bacteria　(AnAOB)　and　nitrite　oxidizing　bacteria　(NOB),　and　backwash　regulated　the　bacteria　community　structure.　After　backwash,　Candidatus　Kuenenia　became　the　dominant　bacteria　and　the　relative　abundance　increased　to　5.56　%.　In　BAF,　one-stage　mainstream　anammox　with　total　nitrogen　(TN)　being　lower　than　15　mg/L　in　the　effluent　was　achieved　using　lag-time　of　bacteria　activity　recovery　caused　by　alternating　operation　of　high　and　low　ammonia　nitrogen　(NH4+-N),　which　have　great　potential　applied　in　municipal　wastewater　treatment　plants　(MWWTPs).