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The oil‐impregnated transformer is an important part of the power system. The ageing degree of the internal oil‐paper insulation determines the service life of the transformer. The low frequency of the frequency domain dielectric spectroscopy can effectively characterise the ageing degree of transformer oil‐paper insulation. For accurately and quantitatively evaluate the aging state of oil‐paper insulation of transformer, the physical process of low‐frequency electrode polarisation is theoretically analysed in this work. Considering the polarisation of the low‐frequency electrode, the theoretical expression of the dielectric response of the equivalent XY model of the transformer is obtained. Meanwhile, to verify the rationality of the theoretical expression, the accelerated thermal ageing test of the oil‐paper insulation samples were carried out under laboratory conditions. The DC conductivity, ion mobility and FDS tests of oil‐paper insulation samples were tested on ageing 5, 10 and 15 days, respectively. The test results were combined with the theoretical expression of the low‐frequency dielectric response. It is found that the difference between the calculated result and the measured result is small. Therefore, the theoretical expression of the transformer equivalent XY model can be applied to calculate the dielectric parameters of the oil‐paper insulation at low‐frequency. [ABSTRACT FROM AUTHOR]