The polyimide/potassium tantalite niobate (PI/KTa0.5Nb0.5O3) nanoparticle composite model is established by a multi-scale modeling method. The influences of KTa0.5Nb0.5O3 nanoparticles with different sizes (5.5, 8.0, 9.4, 10.5, 11.5 Å) on the structure, elastic modulus and interaction energy of the polyimidebased nanocomposites are investigated by the molecular dynamics simulation. The cell parameters, cohesive energy density, solubility parameter, Young’s modulus and Poisson’s ratio are calculated. Moreover, the bond energy and the number of atoms per unit surface area of the nanoparticles are analyzed to explore the internal mechanism of mechanical property improvement. The results demonstrate that the density of PI matrix is 1.24-1.35 g/cm3, the cohesive energy density of PI matrix is 2.025×108 J/m3, and the solubility parameter of PI matrix is 1.422×104 (J/m3)1/2, which are consist with the actual PI parameters. Meanwhile, the Young’s moduli of the PI and PI/KTa0.5Nb0.5O3 composites are respectively 2.914 GPa and 3.169 GPa, and the Poisson’s ratios are respectively 0.370 and 0.353, which illustrate that the mechanical properties of the PI could be significantly improved by introducing the KTa0.5Nb0.5O3 nanoparticles. At the same pressure, the increases of Young’s modulus with temperature are basically the same without and with doping the KTa0.5Nb0.5O3 nanoparticles into the PI matrix; and when the temperatures are different, the standard deviations of elastic moduli of the PI matrix and PI/KTa0.5Nb0.5O3 composite are almost the same. No matter what the pressures and the temperature are, the Young’s modulus of PI/KTa0.5Nb0.5O3 composite is always larger than that of PI matrix. These all indicate that the effect of KTa0.5Nb0.5O3 nanoparticle on elastic modulus has a similar variation rule under the selected pressure and temperature conditions. In addition, the bond energies of particle surface atoms are 8.62-54.37 kJ·mol-1, which shows that the binding force between particles and the matrix is mainly van der Waals force, and hydrogen bonds exist at the same time. When the doping concentration is fixed, the proportion of nanoparticles surface atoms increases significantly as the size decreases, the interaction between particles and the matrix becomes stronger, the Young’s modulus increases obviously and the size effect is more significant. Therefore, it is confirmed that the doping small size KTa0.5Nb0.5O3 nanoparticles into the polyimide matrix is an effective way to improve the mechanical properties of the composite.