Oil-immersed power transformers are critical components in power systems, where their reliability directly impacts system operational safety and stability. During long-term operation, oil-pressboard insulation gradually deteriorates under thermal, electrical, mechanical and the other factors, leading to a progressive decline for the insulation performance. Nano-modification technology significantly enhances the electrical properties of oil-pressboard insulation to improve the reliability of power equipment. Molecular dynamics (MD) simulation technology can offer a powerful approach to investigate the aging micro mechanisms for the oil-pressboard insulation. This paper constructed the oil-immersed pressboard hybrid system and the nano-SiO₂ modified oil-immersed pressboard hybrid system by using the molecular dynamics method. Through the accelerated thermal-electric coupling aging test, the pyrolysis mechanism and the diffusion characteristics of small molecules of the insulation oil-immersed pressboard under high temperature and electric field were systematically studied. The research revealed the influence of the diffusion of water, acid-aldehyde and gases decomposition products generated during the aging process of oil-immersed pressboard insulation pressure plates on their insulation performance. It verified the positive role of nano-SiO₂ particles in improving the anti-aging performance of insulation oil-immersed pressboard. This work deeply explored the pyrolysis mechanism and diffusion behaviors of insulation oil-immersed pressboard from a microscopic perspective, and provided theoretical support for improving the anti-aging performance of cellulose paper and promoting nano-modification technology.