The conductivity of oil-paper insulation materials exhibits significant dependence on electric field strength and temperature. The direct current electric field distribution of the main insulation in converter transformers is influenced by this factor. Currently, manufacturers typically use fixed oil-paper conductivity parameters during the design optimization process of converter transformer insulation, which can lead to deviations in electric field calculations. This paper proposes an improved method for calculating electric fields by fitting the relationship between material properties and variations in electric and temperature fields based on experimental data and principled analysis. The results indicate that under the direct current withstand voltage test conditions, the maximum electric field strength in the insulating oil increases significantly, with a variation ratio of up to 133.2%, while the maximum electric field strength in the insulation paper decreases slightly, with a reduction ratio of approximately 1.7%.