Abstract: Ion accumulation at the oil-paper interface is critical in determining the insulation of converter transformers. Using insulating oils with low ion mobility can suppress accumulation. Vegetable oil's ion mobility is much lower than mineral oil, but the fundamental mechanism is unclear. Reversal polarity experiments and molecular computing are conducted to investigate insulating oils' ion mobility mechanism. Results show vegetable oil's ion mobility is 10-fold lower than mineral oil under converter transformer temperatures. Molecular computing has found self-assembled ion-vegetable oil clusters, where ions adsorb vegetable oil's high electrostatic potential surface, restrict ion mobility. With weak or moderate electric fields (≤1.0V/nm), vegetable oil reduces ion accumulation by decreasing ion mobility. With strong electric fields (> 1.0V/nm), vegetable oil's suppression capability weakens due to cluster decoupling.