<p>Photocatalytic uranium extraction can surpass the intrinsic capacity limits of adsorption by converting soluble uranyl into insoluble uranium peroxides via H₂O₂ generation. However, this process requires simultaneous local enrichment of H₂O₂ and UO₂²⁺ beyond the solubility product constant, which is difficult to achieve in continuously flowing and ultra-dilute seawater. Here we report a hollow hierarchical covalent organic framework (COF) microcavity reactor, HH-COF-(CN/AO)_x, that spatially decouples H₂O₂ generation from uranyl capture to overcome this thermodynamic barrier via a bidirectional reactant flux coupling strategy. A cyano-functionalized inner layer produces and stores H₂O₂ in a central cavity, whereas an amidoxime-rich outer layer selectively enriches uranyl ions. The convergence of outward H₂O₂ flux and inward uranyl adsorption establishes a persistent high-concentration interface that supports continuous formation of insoluble uranium peroxide. The optimized HH-COF-(CN/AO)_0.35 achieves 25.1 mg g⁻¹ uranium uptake in natural seawater, 3.9 times that of the non-hollow analog, which demonstrates a generalizable strategy for manipulating reactant fluxes in ultra-dilute environments for realizing effective resource extraction from water environment.</p>