Abstract: The char samples were prepared from pyrolysis/gasification of Shengli lignite in N₂/O₂/H₂O/H₂O+O₂ atmosphere using a one-stage novel fluidized-bed/fixed-bed quartz reactor, and characterized by BET, Raman, FT-IR, microwave digestion ICP-AES, and TGA techniques. Effects of oxygen addition on gasification reaction and the char structure and reactivity were studied to investigate the activation and mechanism of O₂ on the char. The results indicate that oxygen addition could change the char structure and improve the char reactivity, thus promoting gasification conversion and yield of H₂ and CO₂. The activation of O₂ on the char mainly includes two aspects. First, with the decomposition of aromatic nucleus by oxygen, the large π bond of aromatic rings is destroyed, and the new functional groups are generated, which could promote the reaction (C+H₂O→H₂+CO). Second, with the gasification going on, the large (≥ 6 rings) aromatic structure is depolymerized into small (3-5 rings) one, and oxygen atoms enter the aromatic nucleus, forming the defect position C-O-C. As a result, the defect degree of char microcrystalline structure increases, the polymerization degree of char microcrystalline structure decreases, and the reactivity and surface adsorption of char is improved, which could promote the reaction (CO+H₂O→H₂+CO₂).