In order to study the migration and transformation of SO3 from circulating fluidized bed boiler (CFB) in the air preheater and flue gas treatment facilities, 10 circulating fluidized bed boilers with installed capacities between 150 MW and 330 MW were measured by controlled condensation method. Ten circulating fluidized bed boilers with installed capacities ranging from 150 MW to 330 MW were measured using the controlled condensation method. It is showed that the SO3 removal rate of the air preheater was affected by the air preheater flue gas exit temperature, which ranged from 6.7 % to 36.5 %, and the SO3 removal rate of the rotary air preheater was higher than that of the tubular air preheater. The SO3 removal rate of cloth bag filter and electric bag filter is above 40 %; the SO3 removal rate of electrostatic precipitator is between 6.1 % and 10.8 %. The SO3 removal rate of wet desulfurization method ranges from 41.7 % to 73.4 %, with the highest SO3 removal rate of double tower double cycle. The SO3 removal rate of off-furnace CFB semi-dry desulfurization reached 89.7 %~97.9 %, and the dry process inside the furnace + off-furnace CFB semi-dry process could reduce the SO3 at the desulfurization outlet to below 0.3 mg/m3 under the tested conditions. The conclusions of the test analysis in this paper can provide reference for SO3 control in circulating fluidized bed boilers and related policy formulation.