It remains a challenge to artificially fabricate fiberswith themacroscopic mechanical properties and characteristics of spider silk.Herein, a covalently cross-linked double-network strategy was proposedto disrupt the inverse relation of strength and toughness in the fabricationof ultratough and superstrong artificial polymer fibers. Our designutilized a strong fishnet-like structure based on immovable cellulosenanocrystal cross-links to mimic the function of the & beta;-sheetnanocrystallites and a slidable mechanically interlocked network basedon polyrotaxane to imitate the dissipative stick-slip motionof the & beta;-strands in spider silk. The resultant fiber exhibitedsuperior mechanical properties, including gigapascal tensile strength,a ductility of over 60%, and a toughness exceeding 420 MJ/m(3). The fibers also showed robust biological functions similar to thoseof spider silks, demonstrating mechanical enhancement, energy absorptionability, and shape memory. A composite with our artificial fibersas reinforcing fibers exhibited remarkable tear and fatigue resistance.