Novel three-dimensional (3D) hierarchical cotton-candy-like CuO microspheres were synthesized by a facile precursor templated conversion method. The precursor was prepared by solvothermal method in ethylene glycol (EG) without the use of any surfactant. The possible formation mechanism of the precursor was proposed and it was found that the synthetic parameters for the precursor such as the ratio of Cu(2+) to urea, the reaction temperature, and the use of EG are crucial for the formation of the cotton-candy-like CuO precursor nanostructures. The cotton-candy-like CuO obtained by calcination were used as an adsorbent for removing As(III) in water. The adsorption isotherm, adsorption kinetics, the effects of competing anions and pH, and the adsorption mechanism were also investigated.