Abstract: In oil-immersed power transformers, both the main and longitudinal insulation consist of oil-impregnated-paper insulation, the aging state of which directly influences the operational efficiency and safety of the transformer. To address the issue of assessing the aging state of oil-impregnated-paper insulation, different aging stages of the insulation samples are evaluated through the response of isothermal relaxation current. Three sets of relaxation time constants τ_i and trap peak density D(τ_i) characteristic parameters are extracted, and their relationship with the aging time of the samples is analyzed. Furthermore, the influence of micro-material changes caused by aging on the relaxation current is examined by integrating the analysis of furan content in the insulation oil and infrared spectroscopy of the oil-impregnated-paper insulation. The results indicate that with the progression of aging, both relaxation time constants τ₁ and τ₂, as well as the trap peak density D(τ_i), show an increasing trend, accompanied by a rise in the furan concentration X_F in the oil. Moreover, there is a strong linear relationship (with a goodness of fit greater than 0.95) between the trap peak density D(τ_i) and the furan concentration X_F in the oil, suggesting that trap peak density can be utilized as a characteristic parameter to assess the degree of aging in oil-impregnated-paper insulation samples.