During the vegetable oil-immersed power transformers operating, the transformer oil flows over the surface of the insulating pressboard which leads to occurs the electrification phenomenon in the oil flow. This phenomenon can lead to electrical field distortion and the partial discharge within the insulation system, thereby damaging the insulation of the equipment. To suppress the oil flow electrification phenomenon, this paper presents the method of modifying the oil-paper insulation. Nanometer TiO2 modified pressboard vegetable insulating oil were prepared, and the modification mechanisms were analyzed. The rotating disc system was adopted to simulate the operating conditions of the main insulation of the transformer. The oil flow electrification current of the modified oil-paper was measured, and its influencing factors were analyzed. By establishing a simulation model of the main insulation of the transformer, the internal electrical field distortion in the oil channel of the transformer after the occurrence of oil flow electrification was simulated. The results indicate that the modification of the insulating pressboard using different mass fractions of nano-TiO2 modified insulating oil can suppress the oil flow electrification. Through the comparison of experimental effects, 1 wt% nano-TiO2 modified cellulose pressboard was comprehensively selected as reliable means for suppressing oil flow electrification. Through finite element analysis, it was verified that compared with the unmodified main insulation structure, the modified one has a significant effect on suppressing the oil flow current and the accumulation of charges on the surface of the pressboard.