Abstract: Coal modification treatment is an important technical means to improve the methane desorption performance of coal and the permeability of coal seam, so as to improve the ability of coal seam gas extraction. Three kinds of quaternary ammonium surfactants were prepared into solution to modify long-flame coal, and the influence of quaternary ammonium surfactants on the key chemical structure of long-flame coal was studied. Based on Fourier transform infrared (FTIR) spectroscopy experiment, combined with peak-splitting fitting method to calculate relevant structural parameters, this study aims to clarify the variation characteristics of key functional groups in coal samples during methane adsorption and evaluate methane desorption capacity. The results show that the treatment of quaternary ammonium surfactants does not change the position of infrared absorption peak, but only affects the absorption peak intensity and the relative content of functional groups. After the treatment of quaternary ammonium surfactants, the oxygen functional groups and condensation degree parameters of long flame coal are reduced. In particular, after benzalkonium bromide solution treatment with a mass fraction of 0.1%, the absorption peak area of aromatic structure decreased by 35.56%, maturity parameter decreased by 96.8%, aliphatic chain length increased by 61.2%, and the aromatic ring condensation degree parameter decreased by 58.56%. By analyzing the relationship between structural parameters and methane desorption capacity, it is found that 0.1% benzalkonium bromide solution has the most significant effect on the desorption capacity of long-flame coal.