Plant-based insulating oils are vital materials to enable a sustainable electric power industry. We have previously reported a newly-developed green insulating fluid with low viscosity, low pour point, and high fire point, which we term “medium-chain fatty acid trimethylolpropane triester” (MTT); however, its further application relies heavily on underlies the long-term aging performance. Here, we continually report the aging characteristics of MTT in comparison to those of FR3 oil. The results show that MTT demonstrates the superior anti-aging ability to the FR3 with relatively slow insulation performance decline. Furthermore, oxidation simulation based on the ReaxFF-CHO reveals that the C−O bond-breaking in the MTT oil molecule requires higher dissociation energy, resulting in twice as first-order reaction activation energy as the triglyceride molecules in FR3. Moreover, in the late stage of the thermal oxygen reaction, the polyhydroxy compounds produced by the oxygen reaction of the triglyceride molecules undergo intermolecular dehydration and condensation, leading to the formation of macromolecular polymerization products and thus ever-increasing viscosity of FR3. In contrast, MTT oil hardly undergoes intermolecular dehydration and condensation reaction, so its viscosity remains almost unchanged during the aging process.