Pyrolysis reaction of lignite is one of the important research directions in lignite utilization. Firstly,Shengli Lignite (SL) was pyrolyzed on a fixed bed for coking in order to analyze the structure evolution and gas formation mechanism of lignite during pyrolysis. The final temperature of semi-coke was determined by using the curve of SL py- rolysis gas generation rate at 800 ℃ . The pyrolytic gases was detected on-line by gas chromatography. Secondly,the characterization of Fourier transform infrared spectroscopy ( FT-IR) of coal char is analyzed. And the FT-IR peak of semi-coke is fitted and calculated. Finally,combining the calculation parameters with the law of pyrolysis gas genera- tion,the mechanisms of gas generation in each reaction stage during pyrolysis heating and the evolution law of coal structure in the process of gas generation were proposed. The results show that SL has abundant functional groups such as hydroxyl,aliphatic hydrocarbon,aromatic ring,carbonyl and ether bond. And the main functional groups in lignite have not changed,when the pyrolysis is below 350 ℃ . Aliphatic oxygen-containing functional groups are decomposed between 350 and 450 ℃ . The C O relative content (C1 ) at the pyrolysis final temperature of 450 ℃ is 78% lower than that of 350 ℃ . When the pyrolysis temperature is between 560 and 800 ℃ ,the pyrolysis reaction is mainly based on the decomposition of oxygen-containing functional groups of aromatic alkyl side chains. And the C—O relative con- tent (C2 ) at the pyrolysis final temperature of 800 ℃ is 27% lower than that of 450 ℃ . At 710 -800 ℃ range,the main pyrolysis reaction is polycondensation. The aromatic condensation (D2 ) of chars at a final pyrolysis temperature of 800 ℃ is 65% higher than that at 710 ℃ . Four kinds of pyrolysis gas generation processes were studied and ana- lyzed that CO2 is mainly derived from the decomposition of carboxyl functional groups of different structures in the mid- dle and low temperature zone. CO is generated in the high temperature,which is derived from the decomposition of the structures of phenol,ether and oxygen containing heterocyclic rings in coal. The decomposition of CH4 is mainly from the aromatic ring or methylene connecting the aromatic rings. The approximate 60% H2 mainly comes from the poly- condensation reaction of aromatic structures in coal at a high-temperature zone.