During the oxy-fuel combustion, rich CO2 is produced in flue gas with varied SOx concentration. In this paper, the characteris⁃tics of sulfur retention in fly ash from different coal in oxy-fuel combustion was studied. The SOx absorption in fly ashes of six coals wastested on the high-temperature drop-tube furnace, in which the flue gas atmosphere set for oxy-fuel or air burning. The influencing fac⁃tors were analyzed. The results show that the sulfur retention amount is very low in bituminous coal ash, which is related to the lack of al⁃kaline earth metal (Ca, Mg) in bituminous coal ash. However, the sulfur retention of lignite ash is significant. The fixed sulfur efficiencyis about 43% for lignite ash after air-firing, as compared with 60% after its oxy-fuel combustion. The sulfur absorption rate is much high⁃er in oxy-fuel flue gas in 1 000 ℃ rather than in 800 ℃ . The temperature window and reaction effect of fly ash sulfur fixation are consist⁃ent with the reaction of SO2 oxidation to SO3. Some alkaline earth metals have combined with Si or Al, etc., and these alkaline earth met⁃als do not have sulfur fixation activity. SO3 formation is effected by both catalytic effect of Fe2O3 and the sulfur-retention behavior in ash.Through both oxy-fuel combustion and dry scrubbing of Ca sorbent added together, it is promising to control SOx emission deeply in furnace.