Abstract: The effects of oxygen on changes in char structure and reactivity during steam gasification of Shengli brown coal were examined. A stainless steel simulated entrained-flow reactor was employed to carry out gasification at 800 and 900 ℃ in 8 atmospheres: pure N₂, 15% H₂O, 25%H₂O and 35%H₂O balanced with N₂; and 1%O₂, 15%H₂O+1%O₂, 25%H₂O+1%O₂ and 35%H₂O+1% O₂ balanced with N₂. The structure of chars was measured by Raman spectrum, and the intrinsic reactivity of chars with air was measured using a thermogravimetric analyzer (TGA) at low temperature (300 ℃). The results show that the effects of oxygen on char structure are different at high temperature (900 ℃) and low temperature (800 ℃) in brown coal steam gasification. The volatilization of alkali and alkaline earth metallic (AAEM) is strongly influenced by char structure. The presence of O₂ during steam gasification has a similar effect on the volatilization of Na to the evolution of structure at different temperatures. Experimental data provided evidence that a mixed H₂O/O₂ atmosphere could promote a decrease in subsequent char reactivity in air at 300 ℃, and that char reactivity could correlate well with the band area ratio, I_(GR+VL+VR)/I_D.