Abstract: Existing standards for oil-immersed power transformer dielectric loss factor and insulation resistance attribution method, without considering the effects of insulation aging and moisture content, so there are significant deviations in the calculation results at low temperatures. Based on the FDS test data and insulation resistance data obtained under laboratory conditions in the wide temperature range of oil-paper insulation, combined with the equivalent dielectric relaxation model of the transformer main insulation, and taking into account the moisture content and insulation aging, an equivalent reduction method for the power frequency dielectric loss factor and insulation resistance of transformer main insulation at low temperature is proposed. The measured transformer frequency loss factor and the calculated insulation resistance value can be normalized to 20 ℃, which provides an important theoretical basis for accurate evaluation of transformer insulation status.