Abstract: Nano zero-valent iron (nZVI) has problems such as agglomeration, passivation and poor transportability, which affect the in situ remediation effect of Cr(VI) contaminated groundwater. To develop a low-cost, green nZVI modified material, a low-cost, green modification for nZVI was developed. The nZVI@BC reaction system was constructed by supporting nZVI with ball-milled biochar (BC) as carrier and then stabilized with carboxymethyl cellulose (CMC). CMC-nZVI@BC was synthesized as a novel high-efficiency, anti-passivation nano-scale remediation material. The nZVI before and after modification was characterized and analyzed, and the effects of CMC-nZVI@BC addition, initial concentration of Cr(VI), pH and temperature and chemical fraction of groundwater on the removal of Cr(VI) by CMC-nZVI@BC were investigated, and the mechanism of Cr(VI) removal by CMC-nZVI@BC was elucidated, and the following conclusions were obtained: (1) The best removal of Cr(VI) by nZVI@BC at the Fe and C mass ratio of 2∶1; the removal rate of 50 mg/L Cr(VI) by 0.6 g/L CMC-nZVI@BC within 3 h reached 99.9%, exhibiting a high removal rate and capacity of Cr(VI). (2) The main mechanism of Cr(VI) removal by CMC-nZVI@BC was reduction and precipitation. (3) In the range of 2 to 10, the pH value had a significant effect on the removal of Cr(VI) by CMC-nZVI@BC, with less effect of temperature. (4) The presence of \mathrmSO_4^2- promoted Cr(VI) removal, while \mathrmHCO_3^- , \mathrmNO_3^- , Ca²⁺, Mg²⁺ and humic acid, all had different degrees of inhibition on Cr(VI) removal. These results suggest that CMC-nZVI@BC can be an effective in situ remediation agent for Cr(VI) removal, offering the possibility of applying nZVI for in situ groundwater remediation.