The　effective　inhibition　or　wash-out　of　nitrite　oxidizing　bacteria　(NOB)　is　the　key　challenge　for　the　application　of　partial　nitritation/anammox　(PA/N)　process　in　domestic　wastewater　treatment.　The　deteriorated　domestic　wastewater　PN/A　system　was　used　to　investigate　the　recovery　effect　of　hydroxylamine　(NH2OH)　concentration　and　dosing　mode　on　it.　The　nitrate　(NO3--N)　production　ratio　of　the　PN/A　system　was　0.90,　which　was　due　to　the　overgrowth　of　NOB.　The　results　showed　that　when　the　initial　NH2OH　concentration　was　10mg/L　in　the　sequencing　batch　reactor　(SBR)　and　NH2OH　was　added　once　a　day,　the　ratio　of　NO3--Nproducted/NH4+-Nconsumed　was　decreased　from　0.90　to　0.11　after　adding　NH2OH　for　20d.　It　indicated　that　NH2OH　(10mg/L)　addition　could　restore　PN/A　process　in-situ.　The　ratio　of　NO3--Nproducted/NH4+-Nconsumed　increased　to　0.15　again　after　stopping　NH2OH　addition　for　59d.　When　continued　to　add　NH2OH　into　reactor　for　5d,　the　PN/A　process　operated　well.　Therefore,　the　intermittent　addition　of　NH2OH　strategy　could　maintain　the　stable　performance　of　the　PN/A　process.　The　real-time　quantitative　PCR　results　showed　that　the　NOB　abundance　was　decreased　continuously　from　(4.52±0.44)×1010copies/g　VSS　(Day　6)　to　(2.30±0.80)×109copies/g　VSS　(Day　157)　after　NH2OH　(10mg/L)　addition.　It　indicated　that　NH2OH　addition　was　beneficial　to　inhibit　and　wash　out　NOB.　©　2019,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.