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The converter transformer is an essential part of the DC transmission system. Compared with the traditional oil-impregnated AC transformer, the main insulation of the converter transformer bears more complex electric field aging stress during long-term operation. The influence of the proportion of AC components in the AC/DC composite electric field on insulation aging is still unclear. Therefore, a combined aging test platform of composite electric field and thermal was built in the laboratory, and accelerated aging tests of oil-paper insulation under different AC/DC ratios were carried out. Through the time-frequency domain dielectric response characteristics of oil-paper insulation, the quantitative relationship between the time-frequency domain dielectric response characteristic parameters and AC proportional coefficient in different aging stages was obtained. The results show that the influence of the AC component on the aging of the oil-impregnated pressboard is more prominent. The maximum relaxation polarization time and the maximum exponential coefficient of polarization-depolarization current (PDC) can effectively characterize the aging of oil-paper insulation. Meanwhile, to accurately assess the insulation state of the converter transformer, this paper established the equivalent dielectric relaxation model for the main insulation structure. A quantitative assessment method for moisture content and aging of oil-paper insulation based on time-frequency domain dielectric response was proposed. The influence of transformer oil conductivity, test temperature, and main insulation structure was eliminated. The effectiveness of this method was verified by comparative tests, the maximum error for DP is 20, and the maximum error for moisture content is 0.15%. The research results of this paper can provide theoretical support for on-site assessment of converter transformer insulation status.