Abstract: SO₃ has negative effects on the boiler performance and operation. In order to study effect of Fe₂O₃ on SO₂ catalysis in fly ash, Experimental study about SO₂ oxidation and SO₂ catalysis were carried out in a one-dimensional reactor. Formation mechanism of SO₃ were studied with density functional theory（DFT）. The results show that SO₃ homogeneous generation rate increases with rising temperature. SO₃ heterogeneous generation rate increases initially and decreases afterwards as the temperature increased in presence of Fe₂O₃, which achieves a maximum 4.35% at 800 ℃. SO₃ generation rate decreases with the SO₂ concentration increasing. With catalysis of Fe₂O₃, SO₃ heterogeneous generation rate is significantly improved. The variation in SO₃ generation rate is not obvious at high O2 concentration. Reaction energy barrier of SO₃ heterogeneous generation catalyzed by Fe₂O₃ is 57.357 k J/mol, which is far less than the energy barrier of SO₃ heterogeneous generation in gas phase. Therefore, Fe₂O₃ has good catalytic effect on SO₂ oxidation.