【Objective】 Reductive dechlorination is a key pathway for the degradation of hexachlorobenzene (HCB) which is a persistent organic pollutant. In anaerobic paddy soils, magnetite (Fe3O4) can enhance the direct dechlorination under the action of iron-reducing bacteria and their interacting microorganisms which have a dechlorination function. The process is characterized by an increased electron transfer rate and enhanced chemical reductive dechlorination of organic chlorinated pollutants by an acceleration in the production of adsorbed Fe(II), which is an effective electron donor. To improve the dispersity of Fe3O4, this study attempted to load nano-Fe3O4 onto biochar (BC) and then clarified the effect of nano-Fe3O4/BC composite material on the reductive dechlorination of HCB in anaerobic paddy soil and their possible mechanisms. 【Method】 First, nano-Fe3O4, BC and nano-Fe3O4/BC composite materials were prepared, and their surface morphologies, crystal structures, and characteristic functional groups were characterized. Then, the anaerobic incubation experiment was conducted in slurry systems with Hydragric Acrisols as the tested soil. The internal relationships between pH, Eh, adsorbed or dissolved Fe(II), and the HCB dechlorination process in the reaction systems were analyzed. 【Result】 Results showed that the dechlorination degradation of HCB was negligible for the sterilized control treatment, indicating that the reductive dechlorination of HCB was mainly completed by microorganisms. The addition of single BC accelerated the reductive dechlorination of HCB by increasing soil pH, enhancing the reducibility of the reaction system, and promoting the formation of adsorbed Fe(II). The addition of exclusive nano-Fe3O4 presented a stronger effect on promoting the reductive dechlorination of HCB than the addition of BC alone. This was mainly because nano-Fe3O4 could significantly enhance the production of adsorbed Fe(II) through dissimilatory Fe reduction and adsorbed Fe(II) as an effective electron donor to accelerate the chemical reductive dechlorination of HCB. Also, the application of nano-Fe3O4/BC composite material presented a stronger effect on promoting the reductive dechlorination of HCB than the single nano-Fe3O4. This was attributed to the larger specific surface area of nano-Fe3O4/BC composite material and better dispersity of nano-Fe3O4 on the BC surface. This was more beneficial for the electron transfer process in the reaction system relative to the exclusive nano-Fe3O4 application. 【Conclusion】 In conclusion, the nano-Fe3O4/BC composite material was a more efficient remediation additive for HCB-contaminated soil compared with single nano-Fe3O4 and BC. In the future, the nano-Fe3O4/BC composite material can be promoted and applied in the remediation and treatment of polychlorinated organic pollutants.