Hydroxylamine　(NH2OH)　as　the　putative　intermediate　for　anammox　ensures　the　robustness　of　partial　nitritation/anammox　(PN/A)　process;　however,　the　feasible　for　NH2OH　addition　to　improve　the　stability　of　PN/A　process　under　low-strength　ammonia　(NH4+-N)　condition　need　to　be　further　investigated.　In　this　study,　the　restoration　and　steady　operation　of　mainstream　PN/A　process　were　investigated　to　treat　real　sewage　with　in　situ　NH2OH　added　in　a　continuous　alternating　anoxic/aerobic　with　integrated　fixed-film　activated　sludge　(A(3)-IFAS)　reactor.　Results　showed　that　the　deteriorated　PN/A　process　caused　by　nitrate　(NO3--N)　built-up　was　rapidly　restored　with　a　distinct　decrease　of　the　NO3--　N-produced/NH4+-N-consumed　ratio　from　28.7%　to　＜10.0%　within　20　days,　after　5　mgN/L　of　NH2OH　was　added　daily　into　the　aerobic　zone　of　A(3)-IFAS　reactor.　After　230　days　of　operation,　the　average　total　nitrogen　(TN)　and　phosphate　(PO43--P)　removal　efficiencies　of　80.8%　and　91.5%,　respectively　were　stably　achieved,　with　average　effluent　sCOD,　NH4+-N,　TN　and　PO43--P　concentrations　reaching　23.1,　2.3,　7.7　and　0.4　mg/L,　respectively.　Microbial　community　characterization　revealed　Candidatus　Brocadia　(3.60%　and　2.92%)　and　Ignavibacteriae　(1.56%　and　2.66%)　as　the　dominant　anammox　bacteria　and　denitrifying　bacteria,　respectively,　jointly　attached　in　the　biofilm_1　and　biofilm_2,　while　Candidatus　Microthrix　(5.17%)　dominant　in　floc　sludge　was　main　responsible　for　phosphorus　removal.　This　study　confirmed　that　NH2OH　addition　is　an　effective　strategy　for　nitrite-oxidizing　bacteria　suppression,　contributing　to　the　in　situ　restoration　of　PN/A　process　and　high　stable　mainstream　nitrogen　and　phosphorus　removal　in　a　continuous　PN/A　process　from　real　sewage.　(C)　2021　Elsevier　B.V.　All　rights　reserved.