Abstract: The mercury speciation and concentration in flue gas before and after selective catalytic reduction (SCR) system of a 630 MW coal-fired utility boiler were measured, while the reaction mechanism and effect of chlorine, ammonia and sulfur trioxide in flue gas on the mercury oxidation by SCR catalyst were studied. Results show that SCR has little effect on the mass concentration of total and particulate mercury. SCR catalyst promotes the oxidation of elemental mercury and has significant impacts on mercury emission characteristics. The oxidation of elemental mercury by SCR catalyst is greatly affected by the chlorine content in coal, and high chlorine-containing atmosphere obtained by burning high chlorine coal benefits the Hg⁰ oxidation in the SCR system. NH₃ is detrimental and SO₃ is promotive to the SCR catalytic oxidation of elemental mercury. Increasing the number of catalyst layers is beneficial to the SCR catalytic oxidation of elemental mercury.