Soil erosion significantly influences the global carbon cycle by redistributing organic carbon (OC) from terrestrial to aquatic systems. Check dams, widely implemented for soil and water conservation, potentially influence carbon dynamics by trapping eroded sediments, but their role in carbon sequestration lacks quantitative assessment. This study combined large-scale survey sampling, radiocarbon analysis, and carbon budget equation in the Wuding River basin-a sub-basin of the Yellow River with the most severe soil erosion-to assess the impact of check dams on the basin-scale carbon cycle and clarify their mechanisms as carbon sinks. We found that check dams intercepted 3.7 Tg of OC during 1970-2020, reducing downstream OC export by 16.7%. More importantly, this sequestration suppressed the decomposition of OC during long-distance fluvial transport by 25.5%. The buried OC showed exceptional stability, with low OC content ranging from 1.95 to 2.35 g kg-1 and a high proportion of mineral-associated OC (81.7%). Radiocarbon dating revealed ancient ages, ranging from 2,350 to 10,860 years. Unique depositional conditions, such as rapid burial, a sand-clay layered structure, and anoxic environments, further inhibit the decomposition of OC. Notably, check dams not only mitigated soil erosion but also transformed erosion-prone areas into effective carbon sinks by reducing emissions and enhancing OC burial. These findings reconcile the soil carbon erosion paradox by demonstrating that soil and water conservation measures can significantly alter regional carbon budgets. Our results emphasize the dual role of check dams in soil conservation and climate mitigation, providing a scientific basis for optimizing their design and deployment to enhance terrestrial carbon sequestration.