Abstract: The ²¹⁰Po-²¹⁰Pb pair is increasingly used as a proxy of quantifying organic carbon export from the euphotic zone. However, disequilibria between ²¹⁰Po and ²¹⁰Pb in mesopelagic water have been poorly studied. Here we present unusual deficiencies of ²¹⁰Po with respect to ²¹⁰Pb in mesopelagic water (200–1000 m) in the South China Sea (SCS). The total particulate matter (TPM) increased by up to 32% in the mesopelagic layer comparing with the euphotic zone. The total ²¹⁰Po/²¹⁰Pb ratio varied from 0.41 to 0.98 with an average of 0.72 ± 0.19, showing an enhanced removal of ²¹⁰Po in mesopelagic water. On average, particulate ²¹⁰Po and ²¹⁰Pb increased by 23% and 32% at the slope stations, respectively. These results indicated that the ²¹⁰Po deficits result from lateral transport, probably via benthic nepheloid layer. Based on the deficiency of ²¹⁰Po, the residence times of particulate ²¹⁰Po were estimated to range from 0.11 to 0.25 year (avg. 0.17 ± 0.07 year), allowing resuspended sediment to disperse over a long range. The export fluxes of ²¹⁰Po varied from 68 to 121 dpm m⁻² d⁻¹ with an average of 96 ± 27 dpm m⁻² d⁻¹, which was 6 times that out of the euphotic zone. Using the ²¹⁰Po deficits, the export fluxes of TPM out of the mesopelagic layer were quantified to vary from 4.19 to 10.20 g m⁻² d⁻¹, revealing a large amount of particles from the shelf to the SCS basin. This study suggests that ²¹⁰Po-²¹⁰Pb could be an effective tracer of tracking particle cycling in mesopelagic water.