To reveal the effects of ultralow-emission (ULE) retrofitting on mercury emission from coal-fired power plants, the migration and transformation of mercury in the same coal-fired unit before and after ULE retrofitting were investigated. The mercury species in flue gas including oxidized mercury (Hg2+), elemental mercury (Hg-0), and particulate mercury (Hg-P) was on-site tested by the Ontario Hydro Method, and that in solid and liquid samples was determined in the lab. The effect of upgrading air pollution control devices (APCDs) on the migration and transformation of mercury was analyzed. Finally, the emission factor of mercury after the whole set of equipment was calculated before and after the ULE retrofitting. According to the test results, the synergistic removal effect of each APCD on mercury was enhanced after ULE retrofitting. An increased amount of mercury is trapped by fly ash and wet flue gas desulfurization gypsum, which is about four times of that in coal, and the mercury concentration in flue gas released into the atmosphere reduced from 3.10 to 0.85 mg/Nm(3). In addition, the conventional pollutants, including nitrogen oxide (NOx), sulfur dioxide (SO2), and particulate matter emission, decreased by 55.1, 62.7, and 78.1%, respectively.