Abstract: The usage of perfluorooctane sulfonyl compounds (PFOS) in foam extinguishing agents is strictly regulated. A quantitative nuclear magnetic resonance spectroscopy (qNMR) detection method based on ¹⁹F was established for the rapid determination of PFOS in fire extinguishing agents. Using potassium perfluorobutylsulfonate as the standard substance, the quantitative analysis of PFOS could be realized by calculating the integral area ratio of the ¹⁹F characteristic peak of -CF₃ in potassium perfluorobutylsulfonate at chemical shift δ −78.94 and the ¹⁹F characteristic peak of -CF₂ in PFOS at chemical shift δ −117.12. The linear correlation coefficient between the percentage content of potassium perfluorobutylsulfonate and the quantitative peak area was 0.995 5, the limit of detection was 0.024%, and the limit of quantitative was 0.080%. The qualitative determination results of 8 foam extinguishing agent products were consistent with the actual labeling, and the content of PFOS in 4 foam extinguishing agent products was 0.106%~1.339%. The research results showed that the method has the advantages of low matrix interference, fast detection speed, and high sensitivity, which could provide a reliable detection method and data support for the regulation of PFOS in foam extinguishing agents.