This　study　investigated　the　rapid　start-up　of　mainstream　partial　denitrification　coupled　with　anammox　(PD/A)　and　nitrogen　removal　performance　by　inoculating　precultured　PD/A　biofilm.　The　results　showed　mainstream　PD/A　in　the　anaerobic-anoxic-aerobic　(A(2)O)　process　was　rapidly　established　within　30　days.　Nitrogen　removal　efficiency　(NRE)　improved　by　23.8　%　contrasted　to　the　traditional　A(2)O　process.　The　mass　balance　showed　that　anammox　contribution　to　total　nitrogen　(TN)　removal　were　maintained　at　37.9　%similar　to　55.7　%,　and　reducing　hydraulic　retention　time　(HRT)　strengthened　simultaneously　denitrification　and　anammox　activity.　The　microbial　community　showed　that　the　dominant　bacteria　such　as　denitrifying　bacteria　(DNBs)　and　glycogen　accumulating　organisms　(GAOs)　both　in　biofilm　and　flocculent　sludge　(floc),　integrating　with　anammox　bacteria　(AnAOB)　in　biofilm　might　lead　to　enhanced　nitrogen　removal.　Overall,　this　study　offered　a　fast　start-up　strategy　of　mainstream　PD/A　with　enhanced　nitrogen　removal,　which　are　valuable　for　upgradation　and　renovation　of　existed　municipal　wastewater　treatment　plants　(WWTPs).