Abstract: Polypropylene (PP), as an environment-friendly thermoplastic material, has great potential in the application of ultra high voltage cable lines. However, in practical applications, the thermal and mechanical properties are difficult to improve in coordination, and the humid environment accelerates the initiation of water branches, which seriously hampers the rapid development of PP. To this end, based on molecular dynamics simulation technology, pure PP, composite model of ZnO/PP with sphericalnanoparticles with radius of 0.4 nm and 0.6 nm and water-containing molecular models were built, and the effects of spherical nano-ZnO on the properties of PP were studied from the molecular level by using the changes of thermal and mechanical properties and other parameters. The results show that nano-ZnO can improve the thermal stability of PP and its mechanical properties, and the lifting effect of ZnO particles with different radii is different. Among them, the glass transition temperature of PP increases by 56 K with nano-ZnO with a radius of 0.6 nm, which can significantly reduce its Young's modulus and shear modulus, and has a good inhibition effect on the movement of PP molecular chain and water molecules. Water molecules significantly increase the average azimuthal shift, reduce the glass transition temperature and mechanical parameters, and enhance the diffusion ability with the increase of temperature. Due to the hydrogen bonding between nano ZnO and water molecules, the movement of water molecules in the composite system is limited, thus maintaining good thermal stability. The research results provide micro-level reference for inhibiting the growth and aging of PP water tree and developing practical and environment-friendly cable materials.