Anaerobic　ammonium　oxidation　(Anammox)　is　a　cog-effective　process　for　treating　highly　nitrogenous　wastewater.　However,　the　fate　of　organic　nitrogen　during　Anammox　treatment　is　still　unclear,　which　limits　its　practical　application.　In　this　work,　the　changes　in　the　quality　of　dissolved　organic　nitrogen　(DON)　in　coal　liquefaction　wastewater　(CLW)　during　Anammox　were　studied　in　relation　to　its　chemical　composition,　which　was　determined　by　Fourier-transform　ion　cyclotron　resonance　mass　spectrometry　(FT-ICR-MS).　The　molecular-level　characterization　of　extracellular　polymeric　substances　(EPS)　in　the　Anammox　sludge　is　also　reported　for　the　first　time　in　this　paper.　The　relative　contribution　of　N-containing　compounds　to　the　total　dissolved　organic　matter　(DOM)　determined　by　summating　the　normalized　intensities　exceeded　30%,　highlighting　the　complexity　of　the　nitrogenous　compounds　in　the　influent.　Additionally,　Anammox　appeared　to　be　better　suited　to　removing　DON　compounds　with　fewer　carbonyl　or　carboxyl　groups,　more　aromatic　structures,　and　higher　oxidative　properties.　Lignin-like　substances　were　verified　as　the　predominant　component　of　N-containing　compounds　in　Anammox　EPS,　followed　by　protein　and　substances　with　condensed　aromatic　structures.　DON　compounds　with　higher　degrees　of　saturation,　lower　molecular　weight,　and　higher　lignin-like　properties　were　more　prone　to　absorption　by　Anammox　EPS.　A　series　of　microbe-mediated　pathways　were　demonstrated　to　be　responsible　for　DON　biodegradation,　which　revealed　the　organic　and　inorganic　nitrogen　removal　mechanisms　in　the　Anammox　reactor.　The　obtained　results　provide　great　support　to　the　ongoing　efforts　to　optimize　the　Anammox　process.