Flower-like hierarchical ZnO structures assembled by porous single-crystalline nanosheets were firstly synthesized by a one-pot wet-chemical method followed by an annealing treatment. Then, polyethylenimine (PEI) was used to modify the flower-like hierarchical ZnO structures followed by anchoring Au nanoparticles (NPs) on their surface through electrostatic interactions. The Au-modified flower-like hierarchical ZnO structures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive spectrum (EDS). It can be found that Au NPs with a mean diameter of 12 nm was uniformly modified on the surface of the porous single-crystalline ZnO nanosheets. The as-prepared products exhibited a good response to acetone with a linear range of 0.5–100 ppb under the optimized operating temperature of 300 °C. The lowest detection concentration was 0.5 ppb, which is the lowest detection limit to our knowledge.