To address the insufficient thermal stability and mechanical properties of polypropylene (PP) film capacitors under high-temperature and high-electric-field conditions, as well as the lack of molecular-scale understanding of their modification mechanisms, amorphous cell models of pure PP and VTS@BNNS/PP composites with filler mass fractions of 1%, 3%, 5%, 7%, and 9% were constructed using Materials Studio. In the models, vinyltrimethoxysilane (VTS) was grafted onto boron nitride nanosheets (BNNS) to form an interfacial transition layer. After geometry optimization, annealing, and NVT/NPT equilibration, the interfacial interaction energy, cohesive energy density, mechanical modulus, thermal conductivity, mean square displacement, fractional free volume, and glass transition temperature were calculated. The results show that VTS grafting enhances the interfacial adsorption and compatibility between BNNS and PP. An appropriate amount of VTS@BNNS can fill the gaps between molecular chains, increase the cohesive energy density, and restrict the motion of chain segments, thereby synergistically improving the mechanical and thermal properties. The composite with 7% filler exhibits the optimal comprehensive performance, with Young’s modulus, thermal conductivity, and glass transition temperature are increased by 42.0%, 83.6%, and 41.6%, respectively, compared with pure PP; however, the properties decrease at the filler content of 9%. The simulation results are consistent with the experimental trend, indicating that the established model has good accuracy and predictive capability, and can provide a guidance for the design of high-performance PP composite films.