The concentration of SOx, HCl, CO, NOx etc. in the flue gas during oxy-fuel combustion is all relatively higher compared with air combustion. Higher SO3 concentration increases the risk of low-temperature corrosion. The higher concentration of HCl and CO can also cause corrosion, and they may play roles on SO3 formation. Thus homogeneous experiment and pulverized coal experiment were done on different furnace system to clarify the effects of HCl and CO on SO3 formation during oxy-fuel combustion. Different inlet concentration of HCl, CO, SO2, H2O, O-2 and CO2 were used to simulate the flue gas of the combustion. The results indicated that HCl inhibited SO3 formation but CO promoted SO3 formation at the experimental temperature. HCl richens the radical pool and might generate HO2 radical which was negative to SO3 formation. CO could react with O-2 to generate more O radical which was vital to SO3 formation and consume HO2 radical when steam existed. The pulverized coal experiment presented a different SO3 concentration at the same input sulfur and steam level, meaning the fly ash and other gas phase components in the flue gas could also influence SO3 formation.