Composite insulators are crucial in power systems, but their long-term stability under complex operating conditions still needs improvement. Nano-SiO₂ has potential as a reinforcing filler. This study employs the molecular dynamics simulation method to construct a model of a nano-SiO₂/methyl vinyl silicone rubber (MVSR) composite and systematically studies its microstructure evolution under the action of thermal, electrical, and mechanical stress. The results showed that the addition of nano-SiO₂ significantly improved the microstructure stability of the nano-SiO₂/MVSR composite, specifically manifested in more stable bond lengths, bond angles, and mechanical property parameters. When the electric field intensity reached 350 kV/mm, the total energy of the nano-SiO₂/MVSR composite decreased by only 23.5 %, which was significantly lower than the 57.3 % decrease observed in pure MVSR, demonstrating its energy stability under the electric field. In addition, the simulations revealed that both composites exhibit a high dynamic modulus in the GPa range, characteristic of a high-strain-rate response. Under mechanical stress, the variation range of the mechanical modulus of the nano-SiO₂/MVSR composite was also smaller than that of pure MVSR, indicating superior mechanical stability. These simulation results suggest that introducing nano-SiO₂ can enhance the structural stability of the MVSR matrix at the microscopic level to cope with external stimuli such as heat, electricity, and mechanical stress. These quantitative findings are corroborated by direct visual evidence from simulation snapshots, which clearly illustrate the conformational changes of polymer chains under different stresses, ultimately providing a microscopic basis for improving the long-term service reliability of composite insulators. This study provides atomic-scale insights into the mechanism of nanofillers in polymer insulation materials and provides theoretical references for the design of high-performance composite insulation materials.