Partial　nitrification　and　partial　denitrification　combined　with　anammox　is　a　promising　process　for　sewage　treatment.　In　this　study,　real　municipal　wastewater　was　treated　in　a　continuous　two-stage　anoxic/oxic　(A/O)　reactor.　External　mixed　sludge　fermentation　products　were　added　in　the　anoxic　zone,　simultaneously　achieving　partial　nitrification　and　partial　denitrification　and　achieving　a　high　and　relatively　stable　accumulation　of　nitrite.　The　maximum　accumulation　rates　of　NO2--N　in　A1.2　and　A2.1-A2.4　zones　of　the　reactor　reached　70%　and　61%-　37%,　respectively,　which　improved　denitrification　efficiency　and　created　conditions　that　supported　the　coupling　of　subsequent　anammox.　The　influent　nitrogen　load　of　the　system　was　0.078　kg/(m(3)center　dot　d),　and　the　mean　influent　and　effluent　total　nitrogen　were　51　and　12　mg/L,　respectively.　The　mean　total　nitrogen　removal　rate　reached　76%.　Further　analysis　revealed　that　Hyphomicrobium　(incomplete　denitrifiers)　and　Nitrosomonas　(ammonia　oxidizing　bacteria)　were　enriched,　which　may　have　facilitated　the　high　nitrite　accumulation.　Moreover,　the　batch　test　showed　that　adding　sludge　fermentation　during　denitrification　significantly　suppressed　nitrite　reduction,　resulting　in　the　nitrite　accumulation.