Selective catalytic reduction (SCR) technology has been widely employed in coal-fired power plants (CFPPs) because of its high efficiency in NO removal. Ammonium sulfate aerosols are the byproducts of the SCR of NO with NH3, which lead to adverse effects such as air preheater blockage and increased particulate matter emissions. In this study, the formation mechanism and emission characteristics of ammonium sulfate aerosols in the simulated SCR flue gas were investigated by thermodynamic calculations and laboratory-scale experiments. The concentration and the size distribution of particles were monitored online using an electrical low-pressure impactor, and the aerosol samples used for composition analysis were collected using a PM2.5/10 impactor. Thermodynamic results indicate that H2SO4 rather than SO3 is the main reactant in the formation of ammonium sulfate and bisulfate. The experimental results showed that the formation of ammonium sulfate and ammonium bisulfate depends on the initial NH3/SO3 molar ratio. Ammonium bisulfate is the principal product formed in a typical SCR outlet flue gas, and its initial formation temperature ranges from 231-260 degrees C, calculated as follows:PNH3PH2SO4 (atm(2)) = 7.9 X 10(13) exp(-234968/RT). The concentration of ammonium sulfates aerosol increased with decreasing temperature. Therefore, the initial formation temperature of ammonium bisulfate was higher than the condensation temperature in the actual air preheater. When the concentration of NH3 was 5 mu L/L, the number concentration of ammonium bisulfate aerosol was 5.23 x 10(6)/cm(3). The aerosol size of both the Na4HSO4 and the (NH4)(2)SO4 particles was centered at 0.01-0.1 mu m, but the NH4HSO4 particles were larger as they could agglomerate by collision. Compared with SO3, NH3 concentrations had a greater impact on aerosol emissions.