Abstract: In order to reveal the sequence of active functional groups, diffuse reflectance tests of Beizao lignite were carried out by in situ Fourier infrared spectrometry, and the proportions of active functional groups in the lignite were obtained. A simplified infrared molecular model of the lignite was built based on the same articulation condition of functional groups, and the model was analyzed by using the quantum chemical method. For the main active functional groups, the pr_oportions of methyl methylene and hydroxyl groups are the largest, followed by carbonyl, carboxyl and aromatic hydrocarbon. The frontier orbital and vibration frequency analysis of the model shows that the vibration locations of functional groups in the experiments are consistent with the calculated ones. Combined with the proportions of functional groups and the activity, it can be concluded that in spontaneous combustion the key functional groups are hydroxyl groups and methyl methylene, and then carboxyl.