Abstract: Effects of H₂O on the catalyst Cu⁺/S₂O₈^2-/γ-Al₂O₃ of DMC synthesis in slurry reactor were investigated under the conditions of 120℃ and 3.0 MPa. The results showed that the introduction of water into reaction system led to quick catalyst deactivation. The catalysts were characterized by the Elemental Analysis, XRD, DTG, Py-FT-IR and NH₃-TPD. The results showed that active component Cu and CuCl, loading on the surface of S₂O₈^2-/γ-Al₂O₃ support, reacted with O₂ and H₂O, then formed Cu₂(OH)₃Cl which was essentially inactive for oxidative carbonylation of methanol to dimethyl carbonate. The catalyst deactivation rate speeded up with increasing water injection.