A dual-network hydrogel was prepared for a smart sensing application. Tannic acid (TA) and polyvinyl alcohol (PVA) was gelated as the first network and sodium alginate (SA) was further cross-linked by Ca²⁺ as the second network. The dual-network structure of the SA/PVA/TA hydrogel was investigated by SEM and XRD. The dual-network was composed of multiple hydrogen bonds between PVA, SA, and TA as well as the ion bonds between SA and Ca²⁺. A large number of hydroxyl groups of TA are combined with the network structure of PVA and the chain structure of SA, which improves the toughness and strength of the hydrogel. Further characterization showed that the conductive hydrogel can adhere well to iron, glass, plastic, and finger joints. The hydrogel with TA showed better mechanical properties. After crosslinking in CaCl₂ solution, the tensile strength and conductivity of the SA/PVA/TA hydrogel increased with the increase in immersion time. The SA/PVA/TA hydrogel soaked for 30 min experienced with the extension of external force, and its sensitivity also changed accordingly; the maximum elongation at break of the hydrogel reached 123.8%, while the tensile strength of the hydrogel reached 0.35 MPa. This conductive hydrogel showed good adhesive and tensile properties when adhering to iron, glass, plastic, and finger joints. Then, the hydrogel was deformed while the impedance curve was recorded. The results showed that the dual-network hydrogel can be used as a strain sensor, which provides the new idea that SA/PVA/TA conductive hydrogel can be used as a conductive flexible sensor.