To enhance the anti-biofouling properties and adsorption capability of poly(amidoxime) (PAO), vinylphosphonic acid (VPA, CH₂CH-PO₃H₂) was polymerized on poly(acrylonitrile) (PAN) surface by plasma technique, followed by amidoximation treatment to convert the cyano group (–CN) into an amidoxime group (AO, –C(NH₂)N–OH). The obtained poly(vinylphosphonic acid)/PAO (PVPA/PAO) was used as an adsorbent in the uptake of U(VI) from seawater. The effect of environmental conditions on the anti-biofouling property and adsorption capability of PVPA/PAO for U(VI) were studied. Results show that the modified PVPA enhances the anti-biofouling properties and adsorption capability of PAO for U(VI). The adsorption process is well described by the pseudo-second-order kinetic model and reached equilibrium in 24 h. Adsorption isotherms of U(VI) on PVPA/PAO can be well fitted by the Langmuir model, and the maximum adsorption capability was calculated to be 145 mg g⁻¹ at pH 8.2 and 298 K. Experimental results highlight the application of PVPA/PAO in the extraction of U(VI) from seawater.