【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 and then clarified the effect of nano-Fe₃O₄/biochar composite material on the reductive dechlorination of HCB in anaerobic paddy soil and their possible mechanisms. 【Method】 First, nano-Fe₃O₄, biochar and nano-Fe₃O₄/biochar 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 biochar 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 biochar 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/biochar 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/biochar composite material and better dispersity of nano-Fe₃O₄ on the biochar 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/biochar composite material was a more efficient remediation additive for HCB-contaminated soil compared with single nano-Fe₃O₄ and biochar. In the future, the nano-Fe₃O₄/biochar composite material can be promoted and applied in the remediation and treatment of polychlorinated organic pollutants.