Abstract: The fast pyrolysis of Huolinhe lignite under CO atmosphere was carried out in a fixed bed reactor. The distribution characteristics of gases, influence of CO on pyrolysis behaviors were investigated by comparative analyses of FT-IR spectra, element content and surface structure property of the char. The results show that the CO participates in the pyrolysis process and changes the pyrolysis behavior. Below 600 ℃, the polarity of CO, which brings out by the lone pair electrons existed in CO molecule, can help to crack the aromatic ring, side chain, ether linkages and aliphatic chain in the char, resulting in the increase in smaller molecular fragments and free radicals. These free radicals can stabilize the fragments produced during pyrolysis, which contributes to the generation of more volatile including H₂, CH₄, CO and CO₂, as well as less char with higher specific surface area and pore volume. Above 700 ℃, the carbon deposition produced by the increasing CO disproportionation reaction can partially cover the surface of the char and block its pore, leading to the decrease in surface area and pore volume of the char as well as inhibiting the diffusion of CO in the pore structure. This effect suppresses the contact and reaction between CO and organic macromolecules of coal, resulting in the decreases in the yield of H₂, CH₄ and CO as well as the increase in the yield of CO₂ due to CO disproportionation reaction.