To improve the thermal stability of the cellulose insulation paper used in power transformers, Al 2 O 3 nanoparticles were used to modify traditional cellulose insulation paper. Molecular simulations and experimental methods were used to analyze the enhancement and the microscopic mechanism of Al 2 O 3 nanoparticles in the thermal aging process of insulation paper. Molecular simulations were performed for an unmodified and a Al 2 O 3 nanoparticle-modified cellulose model to explore the changes in mechanical properties, cohesive energy densities and the glass transition temperatures of the two models in a temperature range of 70-150 °C. The results of these simulations showed that the elastic modulus of the cellulose model modified with Al 2 O 3 nanoparticles was larger than that of the unmodified model. Further, the addition of Al 2 O 3 nanoparticles increased the glass transition temperature, signifying that the number of hydrogen bond connections was increased and the thermal stability of the modified model was improved. In addition, physical samples of modified insulation papers mixed with Al 2 O 3 nanoparticles were made, and accelerated thermal aging experiments on these samples showed that the decrease in the degree of polymerization for the modified insulation paper was slower than that of the unmodified insulation paper. Further, breakdown tests suggested that the addition of Al 2 O 3 nanoparticles has a certain positive effect on the AC breakdown voltage. Both the simulated and experimental results indicate that the addition of Al 2 O 3 nanoparticles improves the thermal aging performance of cellulose insulation paper.