||Sulfur trioxide (SO3) is not only environmentally harmful but also highly corrosive, taking a great threat to the safe operation of coal-fired power plants. A dominant pathway of SO3 formation in coal-fired power plant is through the catalytic oxidation of SO2 (SO2+1/2O2→SO3) on the surfaces of ash particles containing Fe2O3. The catalytic formation of SO3 could be affected by complex atmosphere, where the effect from H2O is still debatable. In this paper, density functional theory (DFT) is employed to explore the reaction pathway of SO3 formation catalyzed by α-Fe2O3 in complex atmosphere containing O, O2, SO2 and H2O. In order to get the stable adsorption sites of these species, the adsorption energy of potential adsorption configurations on the α-Fe2O3 (001) surface is calculated. The dissociations of O2 molecule on complete and defect α-Fe2O3 (001) surfaces with O vacancy are calculated, and the Langmuir-Hinshelwood and Eley-Rideal mechanisms for the O(ads) reaction with SO2(ads) or SO2 are compared. The effect of H2O besides of SO2 and O2 on the formation of SO3 is especially discussed. The DFT calculation results show that for the formation of SO3 in gas phase, the energy barrier of ‘SO2+1/2O2→SO3’ is 436.75 kJ mol−1, in contrast, for the catalytic formation of SO3 on α-Fe2O3 surfaces, this energy barrier becomes an order of magnitude smaller, 24.82 kJ mol−1. O2 molecules can dissociate on the defect α-Fe2O3 (001) surface with O vacancy spontaneously, indicating that the defect α-Fe2O3 is favorable for the dissociation of O2, thereby promotes the formation of SO3. The energy barrier of ‘SO2(ads)+O(ads)→SO3(ads)’ through Langmuir-Hinshelwood mechanism is much higher than that of ‘SO2+O(ads)→SO3(ads)’ through Eley-Rideal mechanism. The adsorption energy on the α-Fe2O3 (001) surface of H2O is much smaller than that of SO2 and O2, indicating that H2O has little effect on the adsorption of O, O2, SO2 and eventually the heterogeneous formation of SO3. The DFT analysis results in this study provide a deep understanding on the reaction pathway of SO3 catalytic formation by Fe2O3.