Abstract: The reaction mechanism of water-gas shift (WGS) on the Fe₃O₄ (001)-B surface was systematically studied by using the density functional theory (DFT) calculation with spin polarization. The results show that for the WGS on the Fe₃O₄ (001)-B surface, three reaction routes including redox, association and regeneration ones coexist, though the redox and association routes may be more important with much lower effective energy barriers. The elementary reaction of H₂ formation is influenced by the concentrations of surface H and O defects; higher concentrations of H species and O defects on the catalyst surface are beneficial to the formation of H₂. These results should be helpful for a better understanding of the WGS reaction mechanism on the iron-oxygen catalyst.