The catalytic oxidation on elemental mercury (Hg-0) to water-soluble oxidized mercury (Hg2+) is a good mercury (Hg) control approach for coal-fired power plants, and Hg2+ can be removed by the existing wet flue gas desulfurization (FGD) device. Oxygen-enriched combustion (OEC) is considered as an optimal option for the post CO2 capture among the clean coal combustion technologies. On the basis of the established bench-scale experimental apparatus, this paper studied the influence mechanism of a high H2O concentration on Hg-0 oxidation in the simulated flue gas in OEC. The results showed that both the high HCl concentration and the high temperature could improve the Hg-0 oxidation in the simulated flue gas above 500 degrees C and with as high as 30% of H2O content in OEC. A key intermediate reaction step was needed to complete the reaction between Hg-0 and HCl. HCl was first oxidized to oxidative Cl by O-2. Then, Hg-0 was further oxidized by oxidative Cl. The high H2O concentration could inhibit this process of Hg-0 oxidation, and the reason was that the high H2O concentration could inhibit the conversion from HCl to oxidative Cl. The inhibitory efficiency of oxidative Cl was affected by both the HCl and H2O concentrations. Added SO2 consumed oxidative Cl in the simulated flue gas with a high H2O content in OEC, which inhibited the oxidation of Hg-0. Hg-0 oxidation efficiency increased obviously in the heterogeneous flue gas with ash in OEC. The inhibitory effect of H2O on Hg-0 oxidation could be weakened in the heterogeneous flue gas in OEC.