Abstract: Isothermal pyrolytic characteristics of PBG at 400℃ under different reaction atmospheres in a horizontal tubular quartz reactor were investigated and compared. Meanwhile, chemical structures of PBG and its pyrolysis solid products were also analyzed with the help of XPS and ¹³C NMR methods. The results indicate that tar yields derived from PBG pyrolysis are 50.71 and 37.45 mg/g under biomass air gasification (BAG) and N₂ atmospheres, respectively, while 11.96 mg/g under BAG+2%O₂ atmosphere, which indicates that the presence of oxygen can inhibit the production of tar. Furthermore, the dominant reaction is characterized as the polycondensed aromatization involving dehydrogenation and deoxygenation in PBG under BAG atmosphere, tending to the formation of heavy organic compounds such as tar. While, surface oxygen-containing organic functional groups can be generated via the oxidation reaction between some surface functional groups of PBG and O₂ in PBG under BAG+2%O₂ atmosphere, inhibiting the aromatization of aromatic clusters and the formation of heavy organic compounds such as tar to some extent. Thus, the introduction of a limited amount of oxygen may be helpful for solving the pipe blockage during hot gas filtration of the raw BAG gas.