Glycidyl methacrylate (GMA) was grafted onto an ultrahigh-molecular-weight polyethylene (UHMWPE) fiber through pre-irradiation grafting, and further modified with diethyl (2-aminoethyl) phosphonate. A novel type of phosphate-functionalized fiber (UHMWPE-g-DEPP) was successfully prepared for the separation and recovery of trace uranium in radioactive wastewater. The chemical structure, surface functional groups, stability, micro morphology, and other physical and chemical properties of the fiber materials were analyzed by X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and scanning electron microscopy. The principal factors affecting the U(VI) adsorption performance of the UHMWPE-g-DEPP fiber were investigated, including pH of the aqueous solution, contact time, temperature, and coexisting ions. The experimental results showed that UHMWPE-g-DEPP can achieve an adsorption equilibrium at room temperature within 8 h and the maximum adsorption capacity (113.2 mg/g) at 25 °C, pH = 5.0, and m/V = 0.2 g/?L. The adsorption kinetic pattern was in good agreement with the pseudo-second-order model, and the adsorption equilibrium follows the Langmuir isotherm model well. The phosphate-functionalized fiber showed good recycling and adsorption selectivity.