In this study, we investigated the synthesis of monodisperse CuO hard and hollow nanospheres with extraordinary visible-light photocatalytic abilities for dye photodegradation. The monodisperse CuO hard nanospheres were constructed with assistance from the water-soluble biopolymer sodium alginate (SA). In alkaline solution, the hard CuO spheres can be gradually corroded by ammonia to form hollow CuO nanospheres under hydrothermal conditions. Their visible-light photocatalytic activities were investigated and the results show that both of the hard and the hollow CuO spheres have great visible-light photocatalytic abilities for the photodegradation of dyes. In particular, CuO hollow nanospheres show extraordinary photodegradation activity towards Methyl blue (MB) and it only takes 1 min for 50 mg of CuO hollow spheres to photodegrade 100 mL of 50 mg L–1 MB solution by using solar light and without the assistance of any equipment while for 50 mg of P25 (TiO2), it takes more than 30 min even under UV irradiation. As promising energy-saving materials, CuO nanospheres would have great application potential in environment protection technology.