To explore the dynamic impact fracture behavior of nanoparticle-reinforced composites, a bottom-up numerical method was proposed and verified through the fracture process simulation of nano-SiO2/epoxy sample in Charpy impact test. At the nano-scale, a parametric micromechanics model having interphase was built. And the effective material properties of the nanocomposites with variant volume fractions were obtained. Based on the homogenization theory, the macro-scale model of impact sample was established. It is demonstrated that this proposed bottom-up method can predict the locations and directions of cracks at macro-scale, and the growth process of rupture can also be visualized dynamically. The impact strength obtained from this method has a good agreement with the measuring results in literature. And this simulation method can also be used as an assistant tool for comparing the crack propagation rate of nanocomposites with variant particle contents.