Partial　nitrification　(PN)　is　still　difficult　to　achieve　in　continuous-flow　bioreactors.　In　this　study,　free　ammonia　(FA)　treatment　combined　with　low　dissolved　oxygen　(DO)　were　applied　to　achieve　PN　in　an　A/O　bioreactor.　Batch　tests　showed　200　mg/L　FA　with　12　h　treatment　time　could　effectively　expand　the　activity　difference　between　ammonia　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB),　with　an　24.86%　falling　of　AOB　activity　and　an　54.34%　decrease　of　NOB　activity.　Furthermore,　AOB　activity　can　be　recovered　in　a　shorter　time　after　once　FA　treatment　than　NOB.　FA　treatment　combined　with　low　DO　could　effectively　establish　AOB　advantage　and　achieved　PN　with　the　nitrite　accumulation　ratio　(NAR)　with　86.95%　and　the　ammonia　removal　efficiency　(ARE)　with　93.11%　in　our　A/O　reactor.　FA　treatment　caused　a　decrease　of　extracellular　polymeric　substance　(EPS)　secretion,　among　which　the　protein　reduction　may　be　a　key　factor　that　prevented　NOB　activity　recovered.　Meanwhile,　Nitrospira　abundance　with　FA　treatment　once　a　day　decreased　by　79.1%,　while　Nitrosomonas　abundance　dropped　by　16.4%　only.　Amo　and　hao　abundance　increased　while　nxr　abundance　reduced　by　71.8%.　The　decreased　abundance　of　Nitrospira　and　nxr　was　the　key　to　achieve　PN.　This　study　demonstrated　critical　regulating　factors　for　PN　achievement　in　continuous-flow　bioreactors　using　FA　and　DO,　and　may　provide　a　new　insight　for　PN　in　engineering　applications.