The solar-blind ultraviolet photodetector can find extensive applications in fire prevention, ozone hole monitoring, navigation, and more, for the advantage of no solar background radiation in the ultraviolet-c band (UVC, 200 to 280 nm) at the Earth’s surface. However, most of the UVC photodetectors reported in recent years, including gallium oxide photodetectors, zinc oxide–based photodetectors, and aluminum nitride photodetectors, suffer from the complex and costive fabrication process, which requires high-temperature material fabrication and high fabrication costs. Here, we report an ultraviolet (UV) photodetector composed of magnesium-doped zinc oxide (MgZnO) photosensitive functional material via a simple low-temperature sol–gel fabrication method. In the study, first, the synthesizing method of the MgZnO photosensitive functional layer is systematically investigated. Then, the optical bandgap change of MgZnO with the doped Mg concentrations is explored. The physical model of the relationship between the Mg-doped concentrations and the optical bandgap of the MgZnO photosensitive functional layer is established by spectroscopic methods. Based on the doping study, a highly responsive MgZnO UV photodetector has been designed and fabricated. The MgZnO UV photodetector shows a response bandwidth of 200 to 280 nm, a high responsivity of 108 mA/W, and rise and fall times as low as 0.4 and 2.4 s, respectively. The work reported here may open an easy, low-cost route for developing UV photodetectors.