Marginal seas constitute essential yet insufficiently quantified components of the global carbon cycle, with their budgets increasingly influenced by anthropogenic activities and climatic changes. In this study, we develop a comprehensive organic carbon (OC) budget for the East China Sea (ECS), the largest continental shelf in the northwest Pacific, by integrating field observations with long-term datasets. Our mass-balance model reveals an OC inflow of 145 Tg C/a (1 T=10¹²), primarily driven by the Kuroshio Current (61%) and the Taiwan Warm Current (29%). The outflow of OC is estimated at 127 Tg C/a, resulting in a net OC accumulation of 18.4 Tg C/a within the ECS. Sedimentary burial accounts for 5.52 Tg C/a of sequestration, representing a major removal pathway particulate organic carbon (POC). The cross-shelf export transfers 33.1 Tg C/a (32.1 Tg C/a of DOC and 1.00 Tg C/a of POC) from the inner shelf to the outer shelf. Notably, the cross-shelf flux exceeds rivers inputs, thereby revealing ocean circulation as the dominant regulator of the carbon cycle. Furthermore, we identify clear evidence of non-steady-state dynamics characterized by OC accumulation driven by intensified eutrophication and elevated primary production (111 Tg C/a). This enrichment enhances cross-shelf OC export, indicating a potential alteration in the regional carbon sink capacity under ongoing climate change and eutrophication. Our findings indicate the ECS as a dynamic carbon hotspot and highlight the necessity of accurately predicting the evolving role of marginal seas within the global carbon cycle.