The damage mechanism of ABS/PMMA/nano-SiO2 composites was studied by both experiments and finite element analysis in this paper. A microstructure-based the Homogenization theory and the surface-based cohesive method was developed to predict the deformation and fracture behavior of ABS/PMMA/nano-SiO2 composite. The constitutive behavior of the elastoplastic-damage in the matrix, the fracture for the nano-SiO2 reinforement, and the traction-separation for interfaces, were simulated in this model. The validity of the modeling results were validated by the agreement of the experiment and the morphology of fracture section with those predicted by the simulation. The numerical results were used to provide insight into the damage mechamisms of ABS/PMMA/nano-SiO2 composites, and the effects of nano-SiO2 strength on ABS/PMMA blends mechanical properties were also discussed in detail.