Abstract: By adjusting the flue gas temperature of low-low temperature economizer in a 660 MW ultra-low emission coal-fired power plant, the mercury content in coal and the mercury concentration of different species in flue gas at inlet and outlet of the electrostatic precipitator (ESP) were measured, so as to study the variation characteristics of mercury speciation in the flue gas under different working conditions. Results show that the flue gas temperature in low-low temperature economizer would affect the distribution of mercury speciation, and when the flue gas temperature drops, the elemental mercury turns into oxidized mercury and particulate mercury. The low-low temperature ESP has a synergistic effect on the removal of both gaseous elemental mercury and oxidized mercury. When the outlet flue gas temperature of low-low temperature economizer is 90℃, the shares of elemental mercury and particulate mercury are the highest at the inlet of low-low temperature ESP, while the mass concentrations of elemental mercury and oxidized mercury are the lowest in the outlet flue gas, indicating an optimal effect of low-low temperature ESP on the removal of mercury, with a removal efficiency reaching 84.4%. The mass concentrations of gaseous elemental mercury and oxidized mercury in the outlet flue gas of low-low temperature ESP are higher than other working conditions when the low-low temperature economizer is out of service.