Coal-fired power plants is the main source for mercury emission. In order to characterize the removal characteristics of mercury，a 660 MW coal-fired unit was taken as an example. The migration and transformation characteristics of mercury in selective catalytic reduction equipment(SCR)，electrostatic precipitator(ESP)，flue gas desulfurization (FGD) and wet electrostatic precipitator(WESP)was systematically studied under 500 MW and 600 MW. The results show that the mercury mass balance rate in test locations are 72.01%-90.05% and 79.14%-96.36% respectively，which verify the accuracy of test results. Flue gas mercury is the main discharge form，followed by ash and gypsum. The proportions of mercury emission from FGD waste water and slag account for a small proportion. The conversion rate elemental mercury to oxidized mercury and particulate mercury by SCR catalysts is beyond 40%，but the total mercury reduction is insignificant，and higher ammonia consumption can reduce the conversion rate. ESP can remove all particulate mercury，but the total mercury removal efficiency is less than 33.73% because of lower removal of elemental and oxidized mercury. Because reduction effect of HSO-3 is stronger for oxidized mercury to element，the elemental mercury concentration increases more obviously under 600 MW condition. And the cumulative removal efficiency of oxidized mercury by FGD and WESP reaches over 91%. Total mercury reduction by all the environmental protection facilities is more than 59%. However，collaborative lowest removal efficiency of elemental and oxidized mercury are 6.16% and 94.12% by downstream facilities of SCR，which indicate that mercury oxidation by SCR is the key to reducing mercury emission. Finally，the total mercury concentration in exhaust gas is less than 3.66 μg/m3，which is below the national emission standard.