The properties of insulation paper immersed in ageing-retardant oil and mineral oil were studied through a 264 days ageing experiment. Measurements of the color of the oil, the furfural concentration in the oil, water concentration, the degree of polymerization, and the acid concentration in the paper were performed. The results show that the ageing-retardant oil cannot only resist the oxidation of the oil itself, but also the ageing of the paper immersed within. The relevant inhibition mechanisms were studied. This paper demonstrates the ageing-retardant oil is capable of bounding water molecules. In addition, the esterification, which can resist the hydrolysis of the insulation paper, was studied through infrared spectrum analysis. Furthermore, this paper proposes that the combination of esterification and the hydrogen bonds will contribute to the anti-ageing ability.