RuO2-based catalysts have attracted great attention in mercury emission control region due to their outstanding catalytic activity and long-term stability. Quantum chemistry calculation was performed to uncover the atomicscale reaction mechanism of Hg-0 oxidation by HCl over RuO2/TiO2 catalyst. The results indicate that Hg-0 adsorption on RuO2/TiO2 (110) surface is controlled by a weak chemisorption mechanism. The 5-fold coordinated surface Ru atom is identified as the active center for Hg(0 )adsorption. HgCl molecule serves as an intermediate connecting reactant state to product state. The weak interaction between HgCl2 and catalyst surface is favorable for product desorption. HCl activation is an 0-assisted surface reaction process in which HCl is oxidized into active Cl atom for Hg-0 oxidation. The heterolytic cleavage of HCl molecule occurs without noticeable activation energy barrier. Hg-0 oxidation by HCl over RuO2/TiO2 catalyst proceeds through two independent reaction channels. The dominant reaction channel of Hg-0 oxidation is identified as a four-step process. Finally, a complete catalytic cycle that can produce the correct stoichiometry was proposed to understand the heterogeneous reaction mechanism of Hg-0 oxidation over RuO2/TiO2 catalyst. The catalytic cycle consists of HCl activation, mercury oxidation and surface reoxidation. Mercury oxidation is the rate-determining step of the catalytic cycle.