Metal Fe is one of the phases existing on iron-based catalysts for a high-temperature water gas shift reaction (WGSR), but research on the activity of metal Fe in WGSR is almost not reported. In this work, the density functional theory (DFT) method was used to systematically study the reaction activity and mechanisms of WGSR on metal Fe (110), including the dissociation of H₂O, the transformation of CO and the formation of H₂, as well as the analysis of surface electronic properties. The results show that (1) the direct dissociation of H₂O occurs easily on Fe (110) and the energy barrier is less than 0.9 eV; (2) the generation of CO₂ is difficult and its energy barrier is above 1.8 eV; (3) H migrates easily on the Fe surface and the formation of H₂ also occurs with an energy barrier of 1.47 eV. Combined with the results of Fe₃O₄, it can be concluded that the active phase should be Fe₃O₄ with O vacancy defects, and the iron-rich region plays an important role in promoting the formation of H₂ in WGSR.