To address the difficulty in quantitatively analyzing the aging state of transformer oil-paper insulation using frequency domain spectroscopy characteristics, an evaluation method is proposed for analyzing the aging state of oil-paper insulation by equivalent aging capacitance. Firstly, the equivalent circuit of a transformer insulation system is constructed using recovery voltage characteristics. Then, the intrinsic relationship between the parameters of the extended Debye equivalent circuit and the frequency domain spectroscopy is analyzed, and the feasibility of time-frequency domain spectroscopy spectrum interconversion is studied. Finally, through simulation analysis, the influence of changes in equivalent circuit parameters on the real part of a complex capacitance spectrum is examined, and the relationship between equivalent aging capacitance and the main time constant is quantitatively analyzed. The results indicate that there is an exponential relationship between the equivalent aging capacitance and main time constant; the smaller the equivalent aging capacitance, the more severe the aging state of transformer oil-paper insulation. From the relationship between the main time constant and the aging state of the transformer oil-paper insulation, the equivalent aging capacitance range corresponding to different transformer insulation states can be determined, providing a new reference for accurately evaluating the aging state of transformer oil-paper insulation.