Abstract: To study the change characteristics of the key functional groups affecting the oxidation process of lignitic coal, the in-situ diffuse reflectance Fourier infrared spectrometer was used to increase the temperature from 30 ℃ to 450 ℃ in air and nitrogen atmosphere. In addition, the peak fitting of the infrared spectrum curve at room temperature was carried out to find out the types and contents of functional groups contained in the coal sample, and then the change rule of each functional group in the process of pyrolysis and oxidation was compared to find out the key functional groups affected by coal oxidation. The results show that the coal samples mainly contained oxygen-containing functional groups, followed by hydroxyl, aliphatic hydrocarbon and aromatic hydrocarbon. The hydroxyl groups affected in the oxidation process were mainly intermolecular association hydroxyl groups, free hydroxyl groups and intramolecular hydroxyl groups were not affected by oxygen, only by temperature. Therefore, the key oxidation functional groups in hydroxyl group were the intermolecular association hydroxyl group. The other key oxidation affected were: six vibration forms of aliphatic hydrocarbon, aromatic -CH and aromatic ring C=C- skeleton, carbonyl group, carboxyl group, anhydride.