The molecular adsorption and CO oxidation on a gold-deposited TiO2 catalyst were investigated by means of molecular dynamics simulation. The results indicate that the molecules (i.e., O2, CO, and H2O) are selectively adsorbed on the specific locations such as gold particle, gold-support perimeter interface, and support surface. The adsorption and dissociation of H2O molecules at the perimeter interface enhance the supply of oxygen, thus promoting the oxidation of CO on the Au/TiO2 catalyst. However, the presence of Cl- ions could significantly impede CO oxidation due to their competition with O2, CO, and H2O for the adsorption sites. A reaction mechanism of CO oxidation is postulated on this basis. The findings are useful in developing a comprehensive picture about CO oxidation on gold-deposited TiO2 and in the design of new gold catalysts with high catalytic activity.