Conventional　polymeric　adsorbents　encapsulate　most　of　their　ligand　groups　(such　as　amidoxime,　AO)　in　dense　structures,　leading　to　problems　such　as　low　adsorption　capacity　for　Uranium　extraction　from　seawater　(UES).　The　uranium　coordination　environment　of　polymeric　adsorbents　has　not　been　positively　identified　even　after　decades　of　research.　Therefore,　a　new　and　functional　Biaxially　stretched　polyethylene　(BSPE)　membrane　with　Polyamidoxime　(PAO)　nanoparticles,　mesoporous　architecture,　and　nano-channels　(PAO-BSPE)　was　fabricated　in　this　study　for　UES,　based　on　the　interfacial　self-assembly　of　axial　grafting　chains.　The　underlying　mechanism　involved　in　the　formation　of　the　PAO-BSPE　membrane　was　elucidated　using　first-principles　computations　of　the　various　reactions.　The　engineered　PAO-BSPE　membrane　exhibited　excellent　hydrophilicity,　good　mechanical　properties,　high　specific　surface　area,　and　contained　mesoporous　architecture　and　nano-channels,　which　facilitated　advantages　in　terms　of　kinetics,　capacity,　and　renewability　in　both　laboratory　experiments　and　marine　field-testing.　After　64　days　of　adsorption　in　natural　seawater,　the　PAO-BSPE　membrane　with　a　low　degree　of　grafting　of　70.5%　exhibited　a　high　uranium-adsorption-capacity　of　12.67　mg-U/g-ads,　which　was　1.77　times　higher　than　that　toward　vanadium.　The　PAO-BSPE　membrane　efficiently　extracted　uranium　in　the　presence　of　high　concentrations　of　co-existing　ions,　with　the　distribution　coefficient　reaching　4.09　x　10(6)　mL/g　in　natural　seawater.　The　total　amount　of　uranium　recovered　by　the　PAO-BSPE　membrane　was　15.0　g　in　terms　of　yellow　cake　during　a　total　submersion　time　of　30　days　in　the　ocean.　The　adsorbent　exhibited　a　long　service　life　of　at　least　12　cycles.　Furthermore,　near-edge　and　extended　X-ray　absorption　fine　structure　spectroscopy　analyses　suggested　that　the　mechanism　of　uranium　adsorption　involved　the　formation　of　a　stable　complex　via　the　linkage　of　uranium　with　two　amidoximes　in　a　cooperative　chelating　fashion.　This　study　presents　a　new　adsorbent　with　improved　efficiency　for　UES,　which　has　significant　implications　for　the　development　of　polymer　adsorbents　with　high　selectivity.