The　realization　of　stable　partial　nitrification　and　advanced　nitrogen　removal　are　not　acquired　effectively　in　conventional　pre-denitrification　biological　nitrogen　removal　processes　treating　domestic　sewage.　Herein,　a　novel　anaerobic/aerobic/anoxic/aerobic　(AOAO)　continuous　plug-flow　reactor,　characterized　with　double　sludge　reflux　and　a　bypass　of　anaerobic　mixed　liquor　conveyed　to　anoxic　zone,　was　first　constructed　to　realize　stable　partial　nitrification　in　treating　domestic　sewage.　The　alternating　anoxic/aerobic　conditions　and　longer　anoxic　sludge　retention　time　might　be　responsible　for　the　partial　nitrification.　Nitrite　accumulation　ratio　reached　89.3　+/-　3.3%　with　the　maximum　activity　ratio　of　AOB　to　NOB　increasing　from　0.72　to　8.17.　A　content　total　inorganic　nitrogen　(TIN)　removal　efficiency　(93.7　+/-　2.2%)　and　effluent　TIN　concentration　(2.9　+/-　0.9　mg　N/L)　were　obtained　after　238　days＇　operation.　Specifically,　nitrogen　balance　of　the　typical　cycle　showed　that　about　30.1%　of　TIN　was　removed　through　simultaneous　partial　nitrification　and　denitrification　(SND)　in　aerobic　zone　and　48.2%　by　endogenous　denitrification　in　anoxic　zone.　The　AOAO　process　is　an　economic　treatment　for　domestic　sewage　with　aerobic　hydraulic　retention　time　(HRT)　of　4　h.　(C)　2021　Elsevier　B.V.　All　rights　reserved.