The uranium level in seawater is ≈1000 times as high as terrestrial ores and can provide potential near-infinite fuel for the nuclear energy industry. However, it is still a significant challenge to develop high-efficiency and low-cost adsorbents for massively extracting uranium from seawater. Herein, a simple and fast method through low-energy consumption sunlight polymerization to direct fabrication of a poly(amidoxime) (PAO) hydrogel membrane, which exhibits high uranium adsorption capacity, is reported. This PAO hydrogel owns semi-interpenetrating structure and a hydrophilic poly(acrylamide) 3D network of hydrogel which can disperse and fix PAOs well. As a result, the amidoxime groups of PAOs exhibit an outstanding uranium adsorption efficiency (718 ± 16.6 and 1279 ± 14.5 mg g-1 of m uranium/m PAO in 8 and 32 ppm uranium-spiked seawater, respectively) among reported hydrogel-based adsorbents. Most importantly, U-uptake capacity of this hydrogel can achieve 4.87 ± 0.38 mg g-1 of m uranium/m dry gel just after four weeks within natural seawater. Furthermore, this hydrogel can be massively produced through low-energy consumption and environmentally-friendly sunlight polymerization. This work will provide a high-efficiency and low-cost adsorbent for massive uranium extraction from seawater.