The initial unit cell models of PI/SiO2Nanocomposite and pure polyimide were built by Materials Studio, the stiffness matrix and mechanical parameters such as Young modulus, shear modulus, bulk modulus and Poisson ratio of the unit cells were achieved after molecular dynamic (MD) optimizations and calculations. Finally, the mechanical properties of pure polyimide and the PI/SiO2 nanocomposite with 5vol.% and 10vol.% nanosilica were compared. The result shows that the mechanical properties of polyimide can be remarkably reinforced by being filled with a low volume faction of silica-nanoparticle.