Abstract: To study the variation characteristics of free groups in coal during the oxygen-free heating process, long-flame coal was selected for thermogravimetric analysis and in-situ Fourier transform infrared spectroscopy experiments. The variation trends of functional groups between adjacent temperature points were analyzed by difference spectroscopy, and the quantitative data of functional groups changing with temperature were obtained by combining the Fourier transform infrared spectroscopy deconvolution technique. Further, the second-order derivative spectra and deconvolution spectra were utilized for segmented and peak-segmented fitting to explore the pyrolysis mechanism and kinetic characteristics of the main functional groups in different temperature intervals. The segmented and peak-segmented fitting research was conducted based on the second-order derivative spectra and deconvolution spectra. The results show that: during the entire heating process, the contents of aliphatic functional groups, hydroxyl groups (−OH), carbonyl groups (C=O), C−O and aromatic C=C in coal all decreased, while the content of aromatic C−H increased. The aliphatic groups decreased due to the volatilization of small molecule substances between 30−427 ℃, conforming to the unidirectional transport mode, and the activation energy was 22.7 kJ/mol; at 427−561 ℃, the single carbon-carbon bond breaks, resulting in a decrease in its quantity, which conforms to the third-stage reaction mode, and the activation energy rises to 82.46 kJ/mol; at 561−800 ℃, it conforms to the first-stage pyrolysis reaction, and the activation energy is 162.3 kJ/mol. The aromatic C=C decreases in quantity between 30 ℃ and 427 ℃ due to the cleavage of oxygen-containing functional groups and the volatilization of small molecule substances; at 427−800 ℃, due to the decomposition of aliphatic functional groups resulting in a decrease in quantity, it conforms to the secondary pyrolysis reaction mode, and the activation energy is 207.23 kJ/mol. The transformation of C−O to C=O and its decomposition between 30 ℃ and 266 ℃ reduces the quantity, which conforms to the first-stage pyrolysis reaction mode; at 266−800 ℃, due to the further reduction of pyrolysis of aliphatic functional groups, which conforms to the secondary reaction mode, the activation energy increases from 39.46 kJ/mol to 52.92 kJ/mol. The hydroxyl group (−OH) decreases in quantity due to water evaporation at 30−180 ℃, and decreases in quantity due to the decomposition of aliphatic functional groups at 180−561 ℃; at 266−800 ℃, this stage conforms to the secondary pyrolysis reaction mode, and the activation energy is 72.31 kJ/mol.