Abstract: Hydrogels with good biocompatibility and high mechanical performance are promising for a wide range of biomedical applications. Catechol-modified poly(ethylene glycol) (PEG-catechol) was synthesized via the reaction of isocyanate with hydroxyl and subsequently with amino groups. The PEG-catechol hydrogels were fabricated by covalent cross-linking of intercatechol. And the alginate-Ca²⁺ (Alg-Ca²⁺) was integrated into PEG-catechol network to construct the double network (PEG-catechol/Alg-Ca²⁺ DN) hydrogel by a facile one-pot method. The chemical structure of PEG-catechol was confirmed using Fourier Transform Infrared Spectroscopy (FT-IR) and proton Nuclear Magnetic Resonance (¹H-NMR). The chemical structure, microstructure and thermal behavior of the hydrogels were characterized by Total Reflection Fourier Transformed Infrared (ATR-FTIR) spectroscopy, Scanning Electron Microscope (SEM) and Thermal Gravimetric (TG). The mechanical performance of the hydrogel was evaluated by the universal testing machine. Results showed that PEG-catechol/Alg-Ca²⁺ DN hydrogels exhibited the significantly enhanced tensile strength and tensile fracture energy when compared with PEG-catechol hydrogels (11.9 kPa vs 191.9 kPa and 24.9 kJ/m³ vs 721.9 kJ/m³, respectively). The high strength and toughness of PEG-catechol/Alg-Ca²⁺ DN hydrogels were mainly attributed to energy dissipation through reversible noncovalent bonds in the Alg-Ca²⁺ network and the double network (i.e., Ca²⁺ coordination interactions in the Alg-Ca²⁺ network and hydrogen bonds between two networks of the DN hydrogels). The biocompatible PEG-catechol/Alg-Ca²⁺ DN hydrogels with good mechanical performance have great potential applications in tissue engineering, especially in load-bearing soft tissues such as tendon, cartilage and ligament.