In this paper, we present a mechanical model describing the visco-hyperelastic properties of short fiber reinforced polymer composites, with the purpose to study the reinforcement and fracture mechanisms. On the basis of the Kelvin–Voigt model and deformation behavior of material, the visco-hyperelastic model is developed. The micro-structure of fibers with different distributions in polymers is taken into account. The position and orientation of the fibers are evaluated by numerical analysis, and the demonstration shows that they have a dramatic effect on the reinforcement mechanisms even for the same volume fraction. Given that the interfacial property is influenced by the interfacial region and mold flow direction, a modified function is introduced to characterize the interfacial bonding strength. Furthermore, the model provides reasonable prediction of the fracture toughness.