The　properties　of　dissolved　organic　nitrogen　(DON)　affect　its　stability　in　river　ecosystems.　This　study　is　the　first　to　evaluate　DON　by　assessing　its　stability　in　the　overlying　water　of　an　urban　river.　The　results　showed　that　CHON　was　the　largest　contributor　(87.5%)　to　the　DON,　with　the　largest　portion　being　molecules　with　one　nitrogen　atom.　The　DON　in　the　overlying　water　of　the　Beiyun　River　is　mainly　composed　of　lignin　(70.43%),　following　by　protein　(14.02%),　carbohydrates　(5.28%),　and　condensed　aromatic　(3.74%).　Compared　with　other　urban　rivers,　the　higher　protein　content　and　lower　lignin　content　of　the　Beiyun　River　increase　the　lability　of　the　DON.　From　upstream　to　downstream,　53%　of　molecules　are　always　stable　in　the　river,　most　of　them　are　lignin　with　highly　oxidized　functional　groups.　The　comparison　of　stable　and　unstable　components　shows　lipids　and　condensed　aromatics　are　unstable　and　susceptible　to　degradation　as　compared　with　lignin.　Molecules　with　a　high　H/C,　low　O/C,　and　double　bond　equivalent　(DBE)　were　removed　the　fastest,　while　those　with　higher　O/C　and　O/N　were　relatively　stable.　Flowing　along　the　river,　biological　processes　have　contributed　to　the　transformation　of　DON　molecules　from　high-N　into　low-N　molecules.　Correlation　analysis　suggests　that　cutting　off　external　pollutants　can　be　helpful　to　control　the　nitrogen　pollution　of　the　Beiyun　River.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.