Partial-denitrification　coupled　with　anammox　(PD/A)　process　is　regarded　as　an　emerging　technology　to　improve　nitrogen　removal　efficiency　during　municipal　wastewater　treatment.　In　this　work,　the　shift　from　low　to　high　nitrite　accumulation　in　one-stage　anoxic　biofilm　reactor　using　PD/A　process　was　studied.　When　dinirogen　gas　(N-2)　was　pumped　into　the　bioreactor　to　remove　CO2,　effluent　nitrite　concentration　sharply　increased　from　0.3　mg　N/L　on　day　10　without　N-2　to　14.6　mg　N/L　on　day　21　with　N-2,　but　the　coupled　anammox　reaction　almost　stopped.　Batch　tests　indicate　that　the　metabolic　process　of　PD/A　underwent　a　notable　shift,　transitioning　from　initially　low　nitrite　(＜0.5　mg　N/L)　to　high　nitrite　accumulation　(13.5　mg　N/L).　Metagenomic　sequencing　demonstrated　that　Thauera　was　associated　with　the　genes　napA,　napB　and　narG,　supporting　the　improved　nitrate-to-nitrite　pathway.　The　shift　in　coupling　mechanisms　demonstrated　that　the　one-stage　PD/A　process　has　significant　practical　potential.