Amidoxime polyacrylonitrile fibers (PAO fibers) were prepared with polyacrylonitrile fibers (PAN fibers) and hydroxylamine hydrochloride through a heterogeneous reaction, which were used for low-concentration uranyl ion adsorption (under 1 mg/L). The as-prepared PAO fibers were characterized using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), thermal-gravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) measurements. Batch adsorption experiments were conducted to explore the best adsorbent concentration, pH, contact time, and temperature. When the adsorbent concentration reaches 0.1 g/L to dispose of a uranium solution of 100 mu g/L, the equilibrium concentration can be decreased under 10 mu g/L. The optimum pH for this adsorption experiment is 5.0, and the adsorption can reach equilibrium in 8 h under 298 K. The adsorption process fit well with the intraparticle diffusion model and pseudo-second-order model. In addition, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) models were used to explain the adsorption thermodynamic process. The adsorption process is spontaneous and endothermic.