Spider dragline silk has the unusual combination of high strength, extensibility and toughness, which outperforms some of the best man-made materials in terms of its mechanical performance. Dragline silk has a semi-crystalline structure consisting of crystalline region of short polyalanine segments that form stiff β-sheet nano-crystals surrounded by amorphous glycine-rich domains, which provides extensibility of the fiber. A 3D finite element model of silk fiber is proposed, which is based on the secondary structure of the Araneus diadematus silk fiber, which takes into account the plasticity of β-sheet crystals as well as the viscous behaviour of the amorphous matrix. The silk fiber model shows the predicted mechanical properties are in excellent agreement with available experimental evidence. Initial randomly distributed crystals in the fiber silk rearrange themselves during deformation, and form lamellar-like arrangement of the phases, which results initial stiffness from initial random arrangement, and high toughness due to lamellar-like arrangement. The proposed continuum mechanics based macroscopic silk fiber model, not requiring any empirical parameters, and contribute towards an improved understanding of silk fiber mechanics during deformation and the source of the toughness of this extraordinary fiber. Hence, it is an efficient model for the design of artificial silk fiber as well as applicable to other composite materials.