A family of double-network hydrogels were prepared by copolymerization of acrylamide(AM) with three carboxyl acid-based monomers, acrylic acid (AAc), itaconic acid (IA) and maleic acid (MA), via a combination of chemical cross-linking and soaking in aqueous solution of metal cations. The structures of the hydrogels were characterized by SEM, the coordination forms of carboxyl group with metal ions were analyzed by FT-IR and TGA, the effective network chain density and molecular weight of the chain length between cross-linking points were evaluated using rubber elastic theory, and the effects of carboxyl acid-based monomers and multivalent cations on mechanical properties of the obtained hydrogels were investigated using electronic tensile machine. It was revealed that the single chemical cross-linked hydrogel had uneven interconnection structures with tensile strength of 3.3 kPa. After soaking in divalent cations such as Ca2 , Fe2 and Cu2 , the –COO- groups formed bidentate coordination, resulting in an increase of tensile strengths to 43, 54 and 22 kPa for Ca2 , Fe2 and Cu2 , respectively. In contrast, –COO- groups formed the chelation-bridging tridentate coordination with Fe3 . So the hydrogel after soaking in Fe3 had the highest thermal decomposition temperature and densest structure. As a result, tensile strength increased to 6123 kPa. In addition, tensile strength of hydrogels by copolymerizing AM with AAc and IA were higher than 4.0 MPa while that copolymerized with maleic acid were only around 1.0 MPa. The introduction of ionic bonds between Fe3 and –COO- not only improves the mechanical properties of the hydrogel, but also endows it with self-healing properties. The tensile strength of healed gel can reach 130 kPa.