Hydrogels with a three-dimensional network structure are particularly outstanding in water absorption and water retention because water exists stably in the interior, making the gel appear elastic and solid. Although traditional hydrogels have good water absorption and high water content, they have poor mechanical properties and are not strong enough to be applied in some scenarios today. The proposal of double-network hydrogels has dramatically improved the toughness and mechanical strength of hydrogels that can adapt to different environments. Based on ensuring the properties of hydrogels, they themselves will not be damaged by excessive pressure and tension. This review introduces preparation methods for double-network hydrogels and ways to improve the mechanical properties of three typical gels. In addition to improving the mechanical properties, the biocompatibility and swelling properties of hydrogels enable them to be applied in the fields of biomedicine, intelligent sensors, and ion adsorption.