The　burial　of　sediment　organic　matter　(SOM)　in　the　estuary　and　shelf　plays　an　important　role　in　the　global　carbon　cycle.　However,　it　is　challenging　to　determine　the　source,　composition,　and　burial　of　SOM　in　the　coastal　sea,　especially　at　the　molecular　level.　This　was　explored　in　the　coastal　area　outside　the　largest　Yangtze　River　of　China　with　multiple　techniques　including　elemental　and　stable　isotopic　analysis,　absorption　spectroscopy,　fluorescence　excitation-emission　matrices　coupled　with　parallel　factor　analysis　(EEMs-PARAFAC),　and　ultra-high　resolution　Fourier　transform　ion　cyclotron　resonance　mass　spectrometry　(FT-ICR-MS).　The　end-member　mixing　analysis　based　on　δ13C　and　δ15N　showed　a　dominance　of　marine　contribution　(up　to　70%)　at　most　stations　while　the　terrestrial　contribution　increased　to　＞55%　nearshore　in　summer　at　a　high　fluvial　sediment　flux.　This　was　consistent　with　the　offshore　decreasing　humic-like　C1　and　C2,　humification　index　(HIX),　%lignin-like　compounds,　and　%CHO　but　increasing　tryptophan-like　C3,　biological　index　(BIX),　%protein-like　compounds,　and　%CHOS　from　EEMs-PARAFAC　and　FT-ICR-MS　analysis.　The　%clay　correlated　positively　with　SOM　content,　HIX,　%lignin-like　compounds,　O/C,　and　modified　aromaticity　index　(AImod)　but　correlated　negatively　with　%C3,　H/C,　and　the　relative　abundance　of　labile　formulas　(MLBL),　while　%silt　showed　contrasting　correlations.　These　results　indicated　the　fine　clay　sediments　adsorbed　more　humified,　aromatic,　oxygenated,　and　terrestrial　compounds　that　were　probably　more　resistant　to　biodegradation　and　thus　had　a　higher　burial　efficiency　than　those　on　the　silty　sediments.　Principal　component　analysis　based　on　SOM　indices　further　revealed　different　characteristics　of　SOM　in　the　nearshore,　northern　offshore,　and　southern　offshore　regions,　which　were　probably　dependent　on　the　delivery　by　local　current　systems.　Overall,　these　findings　contributed　to　unraveling　the　source　and　molecular　composition　of　SOM　associated　with　different　grain　size　sediments　and　local　current　delivery,　which　are　fundamental　for　understanding　the　factors　underlying　carbon　burial　in　the　complex　coastal　environment.　©　2024　Elsevier　Ltd