Abstract: Among oil-immersed transformers, partial discharges might be easily developed at the interface of insulation oil and pressboard, which will lead to the failure of transformer's insulation. To explore the improvement of three-element mixed insulation oil on creeping discharge voltage of solid/liquid system, we investigated the characteristics of creeping discharges under AC stress in the system of the three-element mixed insulation oil and pressboard, including the characteristics of creeping discharge voltage, damage of pressboard and gases dissolved in insulating oil, with mineral oil as a reference. It is shown that the creeping discharge voltage of three-element mixed insulation oil and pressboard system increases by 10%~20% compared to that of the system of mineral oil and pressboard, with the electrodes gap distance varying from 5 mm to 20 mm. The main reason is that the electric norm at the needle electrode's tip in the system of three-element mixed oil and pressboard is much smaller than that in mineral oil. Besides, the breakdown performance of three-element mixed insulation oil is better than that of mineral oil. After multiple creeping discharges, damage on three-element mixed insulation oil immersed pressboard is much slighter. The content of C₂H₂ is much less dissolved in three-element mixed insulation oil than in mineral oil, while the total content of hydrocarbon gases has the same characteristics. The results of this paper provide a data support for the securely application of the three-element mixed insulation oil.