Abstract: The effect of the wastewater evaporation in flue duct to the flue gas dew point was studied in this article. Pilot test of the wastewater evaporation was conducted on a 600 MW unit. The H₂O and SO₃ contents in the flue gas were detected and the dew point was calculated using different formulas. The results showed that, when 900~1 400 L/h wastewater was injected in 1/4 flue duct of the 600 MW unit (the amount of flue gas was 4×10⁵~5×10⁵ m³/h), the water vapor in the flue gas increased 0.7%~0.9%, whereas the SO₃ decreased by 30%~50%, which indicated that the transformation from SO₃ to sulfuric acid mist and their adsorption on fly ash particles were improved after the wastewater evaporation. The flue gas dew point decreased by 9~14 °C after the evaporation, and the flue gas temperature could be higher than the dew point. Thus, wastewater evaporation in flue duct could not lead to low-temperature corrosion of the downstream ducts and equipments under the condition of pilot test.