This study investigated the damage mechanism of acrylonitrile-butadiene styrene (ABS)/poly(methyl methacrylate) (PMMA)/nano-SiO2 composites. The distortion and fracture behavior of ABS/PMMA/nano-SiO2 composites were predicted by developing the homogenization theory and the surface-based cohesive method. This model simulated the elementary action of elastoplastic damage in the matrix, the fracture for the nano-SiO2 reinforcement, and traction separation for interfaces. The experiment and morphology of the fracture section were validated by the modeling results. The damage mechanisms of ABS/PMMA/nano-SiO2 composites were researched with the numerical results, and the roles of nano-SiO2 strength in the mechanical properties of ABS/PMMA blends were discussed in detail.