The weak mechanical properties of hydrogels due to the inefficient dissipation of energy in the intrinsic structures limit their practical applications. Here, a double-network (DN) hydrogel has been developed by integrating an ionically cross-linked agar network, a covalently cross-linked acrylic acid (AAC) network, and the dynamic and reversible ionically cross-linked coordination between the AAC chains and Fe3+ ions. The proposed model reveals the mechanisms of the improved mechanical performances in the DN agar/AAC-Fe3+ hydrogel. The hydrogen-bond cross-linked double helices of agar and ionic-coordination interactions of AAC-Fe3+ can be temporarily sacrificed during large deformation to readily dissipate the energy, whereas the reversible AAC-Fe3+ interactions can be regenerated after stress relief, which greatly increases the material toughness. The developed DN hydrogel demonstrates a remarkable stretchability with a break strain up to 3174.3%, high strain sensitivity with the gauge factor being 0.83 under a strain of 1000%, and good 3D printability, making the material a desirable candidate for fabricating flexible strain sensors, electronic skin, and soft robots.