Hydrogel-based water pipes which can capture ionic contaminants are a promising solution for achieving efficient water treatment. However, the fabrication of such unique water pipes as an ion-harvester remains a challenge. In this work, we have fabricated this kind of water pipe through the confined synthesis of coordination frameworks inside a double-network PAAm/PAMPS hydrogel. The hydrogel could trigger the partial decomposition and reduction of K₃[Fe(CN)₆] upon heating. The released Fe³⁺/Fe²⁺ ions could react with the [Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻, finally producing Prussian Blue coordination frameworks inside the hydrogel. The resulting composite exhibited a high capacity for Cs⁺ ions (397 mg g⁻¹ in 10 minutes) by taking the coupling effect between the cation-selective hydrogel and the coordination frameworks. By shaping this composite into water pipes, Cs⁺ ions present in the contaminated water could be captured by the pipe wall. The proposed strategy will be useful, providing a potential method for fast treatment of aqueous nuclear waste.