A multiscale molecular simulation framework integrating density functional theory (DFT), molecular dynamics (MD), and Monte Carlo (MC) methods is employed to elucidate the molecular mechanisms underlying the synergistic enhancement of electrical insulation, oxidative resistance, and thermal stability in polypropylene/styrene-ethylene-butylene-styrene (PP/SEBS) composite via covalent grafting of a tailored voltage stabilizer (3-amino-5-chlorophenyl 3-fluorophenyl methanone, ACFM). First-principles calculations demonstrate that ACFM grafts efficiently introduce multiple charge traps—specifically, electron traps (0.4–2.2eV) and hole traps (0.5–1.6eV)—within the electronic bandgaps of PP macromolecule and PP/SEBS interfacial region, effectively suppressing charge carrier transport. The delocalized π-conjugated system of ACFM phenylene moieties facilitates the dissipation of hot-electron kinetic energy through carrier–phonon coupling, thereby inhibiting impact ionization and enhancing intrinsic dielectric breakdown strength. MD and MC simulations further reveal that the grafted multi-dipolar ACFM side chains enhance thermodynamic compatibility between PP and SEBS segments, promote densification of the amorphous phase, and significantly reduce fractional free volume and oxygen permeability. Consequently, oxygen adsorption capacity and self-diffusion coefficients decrease markedly across the operational temperature range (300–600K), effectively suppressing oxidative penetration and free-volume-mediated electrical breakdown. Reaction pathway calculations indicate that ACFM-grafted PP exhibits approximately 50% reduction in exothermic heat of oxidation compared to pristine PP and SEBS, accompanied by a slight increase in activation energy, thereby corroborating the enhanced antioxidative stability. The covalent immobilization of ACFM thus orchestrates a concerted amplification of charge trapping capability, thermal endurance, oxygen barrier performance, and oxidative resistance, establishing a comprehensive molecular design paradigm for advanced polymer dielectrics operating under harsh environmental conditions.