Abstract: The adsorption characteristics of Hg⁰, HgCl and HgCl₂ on the Mo-doped Fe₃O₄ (111) Fe_tet surface were investigated by density functional theory (DFT) calculation with the CASTEP software package. The results indicate that both HgCl and HgCl₂ are chemically adsorbed on the Mo-doped Fe₃O₄ (111) Fe_tet surface, whereas Hg⁰ is bound to the surface by physisorption. The binding energies of HgCl on the Mo-doped Fe₃O₄ (111) Fe_tet surface is about 40%-66% higher than that on the pure Fe₃O₄ (111) Fe_tet surface. For the adsorption of HgCl₂ molecule on the pure Fe₃O₄ (111) Fe_tet surface, two Cl atoms interact with one Mo atom and one Fe atom, forming the "M" structure; in contrast, on the Mo-doped Fe₃O₄ (111) Fe_tet surface, the stronger interaction between Cl atom and Mo atom allows a complete dissociation of HgCl₂ and release of Hg. The adsorption mechanism of mercury species on the Mo-doped Fe₃O₄ (111) Fe_tet surface revealed in this work may be helpful for the practical removal of mercury from coal-fired flue gases.