Abstract: The conversion rate of SO₂/SO₃ is 0.5%~5% in boiler combustion, but it can be controlled within 1% in SCR DeNO_x in coal-fired power plant. In operation, to control the consumption of SO₃ in the sub-layer of SCR catalyst and air pre-heater in operation, the condensation adsorption quantity of H₂SO₄ and NH₄HSO₄ should be reduced as much as possible, which is aimed at protecting the catalyst activity and ensuring the safety of air pre-heater operation. Theoretically, the low-low temperature electrostatic precipitator has the excellent removal efficiency for SO₃. However, the efficiency may changes from 1.8% to 96.6% in different projects. Study shows that the electrostatic-bag filter has removal efficiency about 74.3%~85.9% for SO₃. Most technologies of wet flue gas desulfurization have removal efficiency of above 50% for SO₃ except the empty absorption tower desulfurization and seawater desulfurization under ultra-low emission condition. Besides, the removal efficiency of wet electrostatic precipitator for SO₃ can be above 50% as well, although there is still a slight gap compared with the mature cases abroad. Overall, the comprehensive removal efficiency for SO₃ of the whole process environmental protection purification equipment of ultra-low emission coal-fired power plants is over 90% and the SO₃ emission concentration of most units are less than 5 mg/m³, when the sulfur content of coal is under 1.5%.