Using a solvent reduction method, Ag nanoparticles were successfully decorated onto ultra-thin porous single-crystalline (UTPSC) ZnO nanosheets which were synthesized through a one-pot wet-chemical method followed by an annealing treatment. The as-prepared products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS) and photoluminescence (PL) measurements. It can be found that the UTPSC ZnO nanosheets with a few micrometers in length and 8-10 nm in thickness were uniformly coated with Ag nanoparticles. The as-prepared products combined the advantages of porous structure, single-crystalline, ultra-thin thickness and Ag decoration, which definitely resulted in a dramatically sensing performance in ethanol detection. The lowest detection concentration was 1 ppb, which is the lowest detection limit to our knowledge. It is expected that the Ag-decorated UTPSC ZnO nanosheets may provide a new pathway to develop advanced nanomaterials for the application of trace gas detection.