Abstract: The adsorption of H3 O+ on clay mineral surface influences the clay-water interface properties,which in turn affects the adsorption of metal ions. The adsorption of H3 O+ on illite (001) and (010) surfaces has been simulated u- sing DFT method. The results show that on illite (001) surface,H2 O adsorbed on Si—O atoms ring easily obtains a proton,and H3 O+ prefers to adsorb above the holes of the Si—O atoms ring. On illite (010) surface,≡Al—OH can get one proton from H3 O+ easily,and there is a competitive adsorption between H2 O and H3 O+ . The electrostatic inter- action of H3 O+ with (001) surface is stronger than that with (010) surface,while the hydrogen bonding of H3 O+ with (001) surface is weaker than that with (010) surface. The adsorption of H3 O+ on the (010) surface is stronger than that on the (001) surface. The presence of H2 O weakens the hydrogen bonding between H3 O+ and the surfaces,but hardly effects on electrostatic interactions. The study concludes that H3 O+ can increase the activity of the interfacial H2 O to promote the interaction of metal cations with the activated interfacial H2 O.