Sulfur-based　autotrophic　denitrification　(SAD)　coupled　with　anammox　is　a　promising　process　for　autotrophic　nitrogen　removal　in　view　of　the　stable　nitrite　accumulation　during　SAD.　In　this　study,　a　mixotrophic　nitrogen　removal　system　integrating　SAD,　anammox　and　heterotrophic　denitrification　was　established　in　a　single-stage　reactor.　The　long-term　nitrogen　removal　performance　was　investigated　under　the　intervention　of　organic　car-bon　sources　in　real　municipal　wastewater.　With　the　shortening　of　hydraulic　retention　time,　the　nitrogen　removal　rate　of　the　mixotrophic　system　dominated　by　the　autotrophic　subsystem　reached　0.46　Kg　N/m3/d　at　an　organic　loading　rate　of　0.57　Kg　COD/m3/d,　with　COD　and　total　nitrogen　removal　efficiencies　of　82.5　%　and　94　%,　respectively,　realizing　an　ideal　combination　of　autotrophic　and　heterotrophic　systems.　The　15NO3--N　isotope　labeling　experiments　indicated　that　thiosulfate-driven　autotrophic　denitrification　was　the　main　pathway　for　nitrite　supply　accounting　for　80.6　%,　while　anammox　exhibited　strong　competitiveness　for　nitrite　under　the　dual　electron　supply　of　sulfur　and　organic　carbon　sources　and　contributed　to　65.1　%　of　nitrogen　removal.　Sludge　granulation　created　differential　functional　distributions　in　different　forms　of　sludge,　with　SAD　showing　faster　reaction　rate　as　well　as　higher　nitrite　accumulation　rate　in　floc　sludge,　while　anammox　was　more　active　in　granular　sludge.　Real-time　quantitative　PCR,　RT-PCR　and　high-throughput　sequencing　results　revealed　a　dynamically　changing　community　composition　at　the　gene　and　transcription　levels.　The　decrease　in　heterotrophic　denitrification　bacteria　abundance　indicated　the　effectiveness　of　the　operational　strategy　for　introduction　of　thiosulfate　and　maintaining　the　dominance　of　SAD　in　denitrification　process　in　suppressing　the　excessive　growth　of　heterotrophic　bacteria　in　the　mixotrophic　system.　The　high　transcriptional　expression　of　sulfur-oxidizing　bacteria　(SOB)　(Thiobacillus　and　Sulfurimonas)　and　anammox　bacteria　(Candaditus_Brocadia　and　Candidatus_Kuenenia)　played　a　crucial　role　in　the　stable　nitrogen　removal.