Abstract: Ontario-Hydro method (OHM) and semi-continuous mercury emission (SCEM) were used to study the effects of the existing air pollutant control devices on mercury emission in coal-fired power plants. The results indicate that more than 50% of Hg⁰ is oxidized into Hg²⁺ by selective catalytic (V₂O₅/TiO₂) reduction (SCR), but SCR itself couldn't control mercury emission. The collection of fly ash by electrostatic precipitator (ESP) directly lowers the proportion of particle mercury in the flue gas. For water soluble Hg²⁺, wet flue gas desulphurization (FGD) could control mercury emission by absorbing Hg²⁺ in flue gas. Most of Hg²+ is reduced into Hg⁰ by reacting with SO₃^2- and HSO₃^- in FGD slurry, which results in the decrease in mercury control efficiency. At the same time, it leads to the secondary emission of mercury, and it is more serious when SCR is in service.