The combustion reaction of raw coals in the air was analyzed withThermal Gravimetric Analyzer 6300 and FTIR (Fourier Transform infrared spectroscopy). The raw coals came from three different sources which were SL lignite, SH bitumite and TT anthracite. The chars were prepared by fixed bed pyrolysis equipment in different reaction temperature. The overlapping peaks were fitted into some sub-peaks by Gaussian function. The aromatic index (R), aromatic structure fused index (D) and organic maturity index (C) were calculated through sub-peaks areas. It showed that three kinds of ignition temperature of SL, SH and TT were 299.3, 408.2 and 441.0 ℃ respectively. The peak temperature of maximum weight loss rate were 348.6, 480.5 and 507.0 ℃ respectively. With the increase of coal rank, both ignition temperature and peak temperature of maximum weight loss rate increased in some degree. The result showed that coal structure was very complex. Vibration absorption peaks of hydroxyl (—OH), aliphatic hydrocarbons (—CH2, —CH3), aromatic (CC), oxygen-containing functional group(CO, C—O) and other major functional groups could be observed in the infrared spectral curves of all samples. With the increase of pyrolysis temperature, infrared vibration absorption peaks of aliphatic hydrocarbons (—CH2—, —CH3) were gradually decreased. the stretching vibration peak of CO which was at 1 700 cm-1 almost disappeared after coked at 550 ℃. SL samples’ absorption peak area infrared curve of oxygen functional groups at 1 000～1 800 cm-1 was more complex. With the increase of coking temperature they changed more significantly compared with others. While peak position and peak intensity for aromatic CC absorption peaks of SH and TT did not change apparently when temperature was changing. Variation trends of main functional groups among three ranks of coals were obviously different with changes of R, D and C values.