An ultrahigh molecular weight polyethylene (UHMWPE) fibrous adsorbent with amidoxime (AO) groups, denoted as AO-UHMWPE, was prepared by preirradiation-induced graft copolymerization of acrylonitrile (AN) and acrylic acid (AA) on UHMWPE fibers, followed by amidoximation. The chemical structure, thermal stability, and mechanical strength were evaluated by means of Fourier transform infrared spectrometry, thermogravimetric analysis, and tensile tests, respectively. The adsorption behaviors of the AO-UHMWPE fiber were studied by batch adsorption in 331 ppb uranium solution, and flow-though adsorption experiments in simulated and natural seawater. It was found that the adsorption conditions (i.e., contact time and manner, temperature, and uranyl ion initial concentration) significantly influence the amount of uranyl ions binding to the AO-UHMWPE fibers. The adsorption of uranium in the batch adsorption experiment was 4.54 g-U/kg-ad in the presence of massive amounts of interference ions.