Abstract: According to the anti-corrosion design and corrosion status of an ammonia flue gas desulphurization (FGD) device in a thermal power plant, the corrosion reasons of the FGD device were analyzed. The results showed that the acid corrosion induced by sulfate ions and chloride ions in desulphurization slurry, the particle crystallization corrosion and mechanical erosion, as well as the low concentration acid dew point corrosion of chimney and other equipment induced by low temperature wet flue gas after desulphurization were the main reasons. The preventive measures of ammonia FGD device were proposed, including reasonable selecting anticorrosion equipment and materials, raising anti-corrosion construction technology standards to ensure anticorrosion construction quality, and controlling corrosion factors in operation to retard the speed of device corrosion.