Abstract: SO_3 is one of the common pollutants discharged from coal-fired power plants,which has a great impact on the stable operation of system equipment and the quality of atmospheric environment. In order to deepen the relevant research,SO_3 morphologic transformation calculation model was established based on thermodynamics and acid dew point theory. The morphologic characteristics of different flue gas temperature,humidity and SO_3 concentration were calculated,and the distribution characteristics of SO_3 morphologic characteristics in flue gas from conventional coal-fired power plants were determined. The results show that SO_3 in flue gas of coal-fired power plants is mainly composed of SO_3 gas,H_2SO_4 gas and H_2SO_4 aerosol. The transformation of SO_3 is based on chemical reaction and physical reaction. The chemical reaction is that SO_3 gas reacts with H_2O gas to H_2SO_4 gas and the physical reaction is that H_2SO_4 gas condenses to H_2SO_4 aerosol by homogeneous and heterogeneous nucleation when flue gas temperature is lower than dew point,respectively. In addition,assuming the valve of humidity is 8%,when the flue gas temperature is lower than 202 ℃,higher than 495 ℃ and lower than 95 ℃,SO_3 exists in the form of H_2SO_4 gas,SO_3 gas and H_2SO_4 aerosol,respectively. Furthermore,combined with the distribution of flue gas humidity and temperature in conventional coal-fired power plants,the distribution characteristics of SO_3 are as follows: SO_3 gas,SO_3 gas and H_2SO_4 gas,H_2SO_4 gas and H_2SO_4 aerosol,H_2SO_4 aerosol exist the section ahead denitrification device,between denitrification device and air preheater,between air preheater and rear section of desulfurization unit,respectively. What is more,the particle size of H_2SO_4 aerosol is less than 0.1 μm.