Abstract: According to the standards of GB/T 7602.3-2008 and DL/T 929-2018, Fourier transform infrared spectrometer was used to detect the structure group composition and antioxidant T501 in insulating oil using KBr absorption cells with optical path lengths of 0.1 mm and 0.5 mm, and the difference of detection results was analyzed. It was shown that the results of structure group composition in insulating oil using the absorption cells with two optical path lengths met the reproducibility requirements from DL/T 929-2018, and there was no significant difference. However, the absorbance of aromatic hydrocarbons and alkane chain hydrocarbons under the optical path length of 0.5 mm was higher, which was suitable for the accurate determination of the structure group composition in insulating oil. The determined results of T501 antioxidant in insulating oil using the absorption cells with two optical path lengths had large deviation, which not met the reproducibility requirements from GB/T 7602.3-2008. However, under the optical path length of 0.5 mm, T501 antioxidant had higher absorbance and better peak shape, and there was no absorption interference, which was more suitable for the determination of T501 antioxidant in insulating oil, and the reproducibility of quantitative results obtained by peak area was relatively good.