Transformer oil and insulation paper are the research objects, accelerated aging experiments in laboratory are performed. The infrared spectrums of samples of different aging time are obtained by the FT-RI apparatus. The absorption intensity of insulation paper’s stretching vibration of O-H at the wave number of 3 346 cm-1 is weaken with the ageing time, so it can be the characteristic quantity of ageing and it can also be the basis for the on-line monitoring of insulation paper in practical transformers. Transformer oil has C=C and C=O group absorption peak at the wave number of 3006 and 1740 cm-1，and the intensity of the absorption peak became stronger with the aging time. The mid-infrared spectra characteristic of the transformer oil-paper is analyzed. The process and mechanism of transformer oil-paper insulation ageing is explored from perspective of microscopic functional groups.