The continental shelf is crucial for the global carbon burial. However, quantifying its POC export is challenging due to the complicated hydrologic conditions and sediment resuspension. Here, seasonal variation of ²³⁴Th and its constraint on POC export were examined in Daya Bay, the northern South China Sea (SCS) Shelf. The total ²³⁴Th showed higher activity concentrations in summer (1.44 ± 0.28 dpm L⁻¹, mean ± sd) and autumn (1.63 ± 0.33 dpm L⁻¹) than those in spring (1.19 ± 0.50 dpm⁻¹) and winter (1.10 ± 0.40 dpm L⁻¹) (t-test, p < 0.05). On average, particulate ²³⁴Th (²³⁴Th_P) accounted for 48% of ²³⁴Th_T. The significant relationships between the total suspended matter (TSM) and ²³⁴Th_P indicated that TSM dominates the scavenging and sinking of ²³⁴Th. The model-results revealed biogenic POC and silica have stronger affinity to ²³⁴Th than lithogenic component, and also represent the main carrier of ²³⁴Th in most cases. Based on the ²³⁴Th deficit, the export fluxes of POC were 11.9 ± 1.7 mmol-C m⁻² d⁻¹, 11.4 ± 1.9 mmol-C m⁻² d⁻¹, 7.0 ± 1.2 mmol-C m⁻² d⁻¹ and 9.9 ± 1.2 mmol-C m⁻² d⁻¹ for Jan.–Apr., Apr.–Aug., Aug.–Oct., and Oct.–Jan. respectively. On the annual timescale, the POC export flux accounted for 15 ± 1% of the primary productivity, higher than the adjacent SCS, highlighting the efficient carbon sequestration in the subtropical Chinese coastal seas.