SO3 generated in coal-fired boiler and selective catalytic reduction (SCR) process has become a significant Problem due to its adverse effects on boiler facilities and air environment. The factors influenced conversion rate of SO2 to SO3 (SO2/SO3) during SCR process were studied and evaluated, which included the effects of flue gas composition and SCR catalyst components on the conversion of SO2/SO3. SO3 was trapped by the condensation and isopropanol absorption method, and SO42- concentration was detected by spectrophotometer. The results showed that a certain concentration of NH3 in the flue gas significantly promoted the conversion rate, but when NH3 and NO coexisted, NO inhibited the catalytic action of NH3, while NO itself and O-2 had small effect on the conversion rate. Besides, the conversion rate of SO2 increased with the increase of Ti and V content in the catalyst, and SiO2 and BaO abated the catalytic oxidation of SO2, while WO3 promoted the formation of SO3.