A simple two-step aqueous method was employed to grow MgO nanostructures on ZnO/sapphire at low temperature. The obtained thin MgO nanostructures were uniformly distributed on the surface of ZnO layer and showed the sheet-like structures. Meanwhile, an ultraviolet (UV) photodetector based on ITO/MgO/ZnO structures was fabricated by simple way. The obtained UV detector showed excellent UV sensing properties. This novel method will greatly facilitate fabrication of large-scale metal-insulator-semiconductor with relatively low cost at remarkably low temperature.