Accurately detecting the concentration of methanol dissolved in transformer oil is of great importance for diagnosing the aging degradation of insulating paper. In this paper, acetonitrile were selected as extractants, and mixed with transformer oil samples containing different concentrations of methanol to complete the sample pretreatment work. The concentration of methanol in transformer oil samples was determined by Raman spectroscopy. The quantitative analysis method based on the principal component analysis of peak intensity of Raman spectra and the quantitative analysis method based on principal component analysis of peak area of Raman spectra were compared. The results show that the accuracy of fitting and error calculation using Raman spectral peak area as quantitative analysis parameter is higher, especially in the low concentration case concerned, it is helpful for us to detect trace methanol dissolved in transformer oil.