Uranium　extraction　from　seawater　(UES)　has　important　strategic　significance　for　maintaining　the　sustainable　development　of　nuclear　energy.　This　article　presents　the　preparation　of　a　low-cost,　efficient,　and　highly　reusable　biosorbent　sodium　alginate/polyethyleneimine　(SA/PEI)　through　a　simple　one-step　crosslinking　process.　The　chemical　crosslinking　between　PEI　and　SA　provides　biosorbent　excellent　mechanical　strength　and　thermal　stability.　SA/PEI　was　characterized　by　using　FTIR,　XRD,　TGA,　EDS,　XPS,　SEM　before　and　after　adsorption　of　uranium.　Thermodynamic　research　results　show　that　the　uranium　adsorption　of　SA/PEI　is　a　spontaneous,　entropy　increasing　endothermic　process.　The　adsorption　fitted　the　pseudo-second-order　kinetic　model　and　Langmuir　model　with　maximum　adsorption　capacity　reach　353.09　mg　g-　1,　illustrating　that　the　adsorption　mechanism　is　monolayer　chemical　adsorption.　The　interaction　between　SA/PEI　and　uranium　is　synergistic　chelation　by　amino　and　carboxyl,　which　is　consistent　with　the　results　calculated　by　DFT.　After　14　days　of　adsorption　in　100　L　natural　seawater,　the　adsorption　capacity　of　SA/PEI　was　3.58　mg　g-　1,　with　an　average　adsorption　efficiency　of　0.256　mg　g-　1　day-　1,　which　is　faster　than　most　reported　alginate　adsorbents.　The　cost　of　using　SA/PEI　to　UES　is　$168　per　kilogram　of　uranium.　These　results　indicate　that　SA/PEI　hydrogel　has　great　potential　in　practical　seawater　application.