A new type of double network hydrogel-based adsorption was prepared by using simple free-radical polymerization of acrylic acid in alginate, and functionalized by the triethylenetetramine. By characterization of the hydrogel, it implied that the hydrogel possessed three-dimensional interpenetrating network structure and fair mechanical stability. In the experiment of adsorption, the hydrogel had a higher adsorption rate when the pH value was greater than 2.5. Cd(II) ion could be effectively diffused owing to the three-dimensional network structure of the hydrogel. Under the condition of the ion concentration being 120 mg/L and the hydrogel mass concentration being 1g/L, the adsorption equilibrium was reached within 60 min. When the ion concentration was lower than 120 mg/L, the adsorption efficiency of the hydrogel reached more than 95% because of a large number of exposed active sites. In the selective adsorption experiment, the hydrogel had the highest adsorption efficiency for Cd(II), and the distribution adsorption coefficient (kd) reached 84.62 L/g. This work developed a highly practical adsorption process based on hydrogel adsorbents for the removal of heavy metals in actual wastewater.