Acrylonitrile and 2-(dimethylamino)ethyl methacrylate were grafted onto ultrahigh molecular weight polyethylene fibers, and a series of quaternary ammonium (Q)-functionalized amidoxime (AO)-based fibers including QAO fibers and AO-Q fibers were synthesized. The optimal preparation process was determined. The results show that the QAO fibers with quaternary ammonium grafting chains in the inner layer and the amidoxime groups in the outer layer exhibited the best adsorption performance. Enhanced adsorption kinetics and adsorption capacities of the Q-AO fibers were obtained in simulated seawater. In comparison with the AO-Q fibers, the Q-AO fibers showed a higher adsorption capacity of 29.73 mg/g, which was 54.7% higher than that of the AO-Qfiber and 108.6% higher than that of the traditional amidoximebased adsorbent (AO fiber). These implied the obvious effect on the distinction of the preparation processes. Compared with AO fibers, the adsorption capacity for uranium by the Q-AO fibers in natural seawater increased nearly seven-fold to 0.210 mg/g, while the capacity for other metal ions decreased. This study concluded that synergistic groups with positive charges could result in enhanced adsorption performance for uranyl tricarbonate from natural seawater, and the distribution distinction of the AO group and synergistic group deeply affect the adsorption performance.