Hierarchical 3D flower-like Cu O nanostructures on the Cu substrates were synthesized by a wet chemical method and subsequent heat treatment. The synthesis, structure and morphologies of obtained samples under different concentrations of Na_2S_2O_3 were investigated in detail and the possible growth mechanisms of the 3D flower-like Cu O nanostructures were discussed. Na_2S_2O_3 plays a key role in the generation of the 3D flower-like Cu O nanostructures. When the concentration of Na_2S_2O_3 is more than 0.4 mol/L, the 3D flower-like Cu O nanostructures can be prepared on the Cu foils. The photocatalytic performances were studied by analyzing the degradation of methyl orange(MO) in aqueous solution in the presence of hydroxide water(H_2O_2). The 3D flower-like Cu O nanostructures exhibit higher photocatalytic activity(96.2% degradation rate) than commercial Cu O particles(36.3% degradation rate). The origin of the higher photocatalytic activity of the 3D flower-like Cu O nanostructures was also discussed. Hierarchical 3D flower-like Cu O nanostructures on the Cu substrates were synthesized by a wet chemical method and subsequent heat treatment. The synthesis, structure and morphologies of obtained samples under different concentrations of Na 2 S 2 O 3 were investigated in detail and the possible growth mechanisms of the 3D flower-like Cu O nanostructures were discussed. Na_2S_2O_3 plays a key role in the generation of the 3D flower-like Cu O nanostructures. When the concentration of Na_2S_2O_3 is more than 0.4 mol / L, the 3D flower-like Cu O nanostructures can be prepared on the Cu foils. The photocatalytic performances were studied by analyzing the degradation of methyl orange (MO) in aqueous solution in the presence of hydroxide water (H 2 O 2). The 3D flower-like Cu O nanostructures exhibit higher photocatalytic activity (96.2% rate) than commercial Cu O particles (36.3% degradation rate). The origin of the higher photocatalytic activity of the 3D flower-like Cu O nanostructures was also discussed.