Salt leakage is a key failure mode of hygroscopic hydrogel in atmospheric water harvesting (AWH), which will lead to a rapid performance degradation and secondary contamination. The fundamental reason is the thermodynamic incompatibility between matrix and high concentration saline water. In this paper, a robust and stable hydrogel is designed through the thermodynamic anchoring strategy at the molecular level. This method involves covalently grafting ionizable groups onto gelatin/starch scaffolds to form specific electrostatic traps that resist strong water binding forces and anchor lithium ions. Density Functional Theory (DFT) calculations confirmed that these anionic sites created a high energy barrier that thermodynamically dominates over the hydration forces of water molecules, thereby locking lithium ions even under saturation. Therefore, the gelatin/ tapioca starch/2-Acrylamido-2-methylpropane sulfonic acid/ lithium chloride (GTA@LiCl) hydrogel achieved high water collection capacity up to 5.2 g g-1 (90% relative humidity). In addition, the collect water meets the agricultural water safety standards in the solar powered water production experiment. This ability makes it possible to irrigate crops in arid environments, extending plant survival by 25 days. This study establishes a scalable chemical paradigm that decouples high hygroscopicity from salinity risk, making biomass based AWH a reliable and safe water supply technology.