The concentrations of total organic carbon (TOC) and total nitrogen (TN) and their isotopic compositions (δ¹³C and δ¹⁵N) were determined in surface sediment (0-3 cm) and sediment cores in the Hanjiang river estuary during the spring, summer and winter cruises in 2021, to elucidate the distribution and source of sedimentary organic matter. The horizontal distributions of TOC and δ¹³C_TOC in the Hanjiang river estuary have a similar pattern, namely, TOC concentration decreased with the increase of offshore distance and the value of δ¹³C_TOC enriched with the increase of offshore distance. Higher TOC concentrations and enrich δ¹³C_TOC were observed in the marine cultivation area of Yuedong. The distribution patterns of TN and δ¹⁵N_TN were in consistent with those of TOC and δ¹³C_TOC, respectively. The observed values of δ¹³C_TOC, δ¹⁵N_TN and C/N ratio constrained the two dominant sources of marine and terrestrial organic matter in the Hanjiang river estuary. Using a two end-member mixing model, we calculated the contribution of the marine organic matter to be 74±16% in summer and 75±12% in winter, respectively. This suggested in situ production of marine phytoplankton was the dominated organic carbon in Yuedong sediment and had no seasonal variability. Additionally, high inventory of TOC in the marine cultivation area of Yuedong indicated higher carbon storage and carbon fixation capacity. Finally, this study helps us to establish a background database for the future study on the peak and neutrality of carbon in China.