The rapid expansion of deep-sea offshore wind power necessitates advanced 100 Hz Modular Multilevel Converters, yet their insulation systems face accelerated aging under coupled thermal-electrical stresses. This study explores the molecular-level degradation mechanisms of oil-pressboard insulation in MMCs using molecular dynamics simulations. A hybrid model of cellulose and hydrocarbon molecules was constructed, optimized through annealing cycles, and subjected to thermal and electrical fields. Key parameters including cohesive energy density, kinetic energy, hydrogen bonds and shear modulus were analyzed. Results show that elevated temperatures and electrical fields synergistically reduce structural stability and increase molecular mobility. Disruption of hydrogen bonds and degradation of the shear modulus further reveal weakened mechanical integrity. These findings underscore the critical role of thermal-electrical coupling in accelerating insulation aging, offering valuable insights for optimizing material design and enhancing operational stability in deep-sea wind power systems.