Abstract: Nuclear energy, as a safe, low-carbon, stable, and efficient clean energy source, demonstrates broad development prospects in global energy transition and climate change mitigation. Uranium is a key raw material for nuclear energy, and its demand continues to grow with the rapid development of the nuclear industry. Seawater contains abundant uranium reserves, nearly a thousand times that of terrestrial uranium. Therefore, uranium extraction from seawater is crucial for the sustainable development of nuclear energy. Currently, there are various methods for seawater uranium extraction, such as adsorption, ion exchange, membrane separation, photocatalysis, and electrochemical methods. However, each method has its own advantages and disadvantages. Combining different methods to leverage their strengths can potentially improve uranium extraction efficiency. In this study, a porous composite aerogel material, PAO/g-C₃N₄, was prepared by first combining adsorbent and photocatalytic materials into a hydrogel through molecular crosslinking, followed by freeze-drying. The material was characterized by SEM, FTIR, XRD, and other techniques. Batch experiments were conducted to investigate the effects of solid-to-liquid ratio, pH, ionic strength, contact time, and initial concentration on adsorption performance. The results show that the prepared aerogel material has a porous network structure, which exposes more active sites and increases the contact opportunities between the material and uranyl ions. Additionally, the composite aerogel PAO/g-C₃N₄ retains the original properties of its components. Adsorption experiments indicate that both PAO/g-C₃N₄ and PAO follow pseudo-second-order kinetics, with optimal uranium adsorption under near-neutral conditions. The uranium adsorption capacity of PAO fits the Langmuir adsorption model, suggesting monolayer chemical adsorption. In contrast, the uranium extraction capacity of PAO/g-C₃N₄ does not reach saturation within the studied concentration range, indicating its excellent uranium extraction capability. The uranium extraction performance of both PAO/g-C₃N₄ and PAO is influenced by pH and ionic strength, suggesting that inner-sphere and outer-sphere complexation may be the adsorption mechanism. Moreover, PAO/g-C₃N₄ exhibits good reusability and ion selectivity. In simulated seawater with extremely low uranium concentration, PAO/g-C₃N₄ shows higher uranium extraction than PAO. Mechanistic analysis reveals that PAO relies solely on adsorption, while PAO/g-C₃N₄ combines both adsorption and photocatalysis. The amidoxime groups in PAO primarily coordinate with uranyl ions, while g-C₃N₄ acts as a photocatalyst. Due to the presence of dissolved oxygen during the experiment, the photocatalytic product of g-C₃N₄ is uranyl peroxide dihydrate. Under the combined effects of adsorption and photocatalysis, PAO/g-C₃N₄ achieves efficient uranium extraction, demonstrating potential for practical applications. Furthermore, this study provides a new approach for the synergistic application of adsorption and photocatalysis.