Abstract: Microplastic (MP) pollution in marine environment has become a global concern. Fiber is reported to be the main form of MPs along the coastal water of China. However, quite few studies have been conducted to explore the impact of microfibers on marine benthic organisms, and the uptake and depuration processes of microfibers by benthic organisms are still unclear. In this study, using sediment-dwelling Perinereis aibuhitensis as model organism, we investigated the uptake and depuration kinetics of polyethylene terephthalate (PET) microfibers (100 µm in diameter and 1.09±0.21 mm in length) under approaching environmentally relevant concentrations (50, 100 and 200 items/L) in P. aibuhitensis through a seawater exposure pathway. The results of uptake experiment showed that for the whole 72 h of exposure process, the ingestion of PET microfiber generally increased and reached the maximum values of 2.35 items/g, 4.89 items/g and 8.55 items/g for the low, medium and high concentration exposure groups at the end of exposure, respectively. The uptake constant K_u of PET microfibers for three concentration groups reached the maximum values of 1.70×10⁻⁴, 1.60×10⁻⁴ and 1.40×10⁻⁴ L/kg/d at 6 h, 1 h and 3 h after exposure, respectively. For the results of depuration experiment, the remaining amount of PET microfibers in P. aibuhitensis body showed a generally decreasing trend and reached the minimum value (1.17 items/g, 2.05 items/g and 2.98 items/g, respectively) in low, medium and high treatment at 72 h, 24 h and 48 h after recovery, respectively. The depuration constant K_d of PET microfibers in the low concentration group reached the maximum value (4.81±1.95)/d at 3 h after recovery and the corresponding residual half-life t_1/2 is (0.28±0.18) d; for the medium and high concentration groups, K_d reached the maximum values of (7.19±4.20)/d and (9.12±3.30)/d, respectively, corresponding to t_1/2 reaching the minimum value of (0.18±0.08) d and (0.11±0.05) d at 1 h after recovery, respectively. At the end of recovery stage, PET microfibers mainly remained in the worm body, followed by seawater solution, and fecal distribution being the least. This study showed that the concentration of PET microfibers affected their uptake and depuration amounts and constant rates in P. aibuhitensis. Our results provide a basis for scientific assessment of the toxic effects and ecological risks of microplastics in benthos.