Uranium (U), a high-performing, low-emission energy source, is driving sustainable economic growth. Herein, we synthesized two crystalline phases (HPOC-α and β) by an unreported amidoxime organic cage used for uranium capture. The revealed crystal structures and uranium adsorption test showed that accessible functional groups were essential to uranyl ions sorption. The amidoxime groups presented in HPOC-α gifted it an equilibrium uranium capacity of 1682 mg g-1 and could mostly maintain the performance after adsorption-desorption cycles. In natural seawater, HPOC-α exhibited an impressive uranium recovery capacity of 11.97 mg g-1 during a 30 day field testing, showing potential for economically practical uranium extraction from seawater.