The invention discloses a gallium oxide X-ray detector and a preparation method thereof and belongs to the semiconductor device manufacturing field. When semiconductors such as silicon, germanium, CdTe, and CdZnTe are adopted as x-ray detectors, the x-ray detectors are sensitive to ambient temperature and are weak in radiation resistance because that the band gap of the materials is small, and asa result, the application of the x-ray detectors to the space field is limited. According to the detector of the present invention, a germanium oxide single crystal is adopted as a substrate; the upper surface of the germanium oxide single crystal is provided with a contact electrode and a Pt/Au working electrode; the lower surface of the germanium oxide single crystal is provided with an ITO/Ti/Au working electrode; the top of the Pt/Au working electrode and the bottom of the ITO/Ti/Au working electrode are connected with an external circuit through connecting leads; and the electrodes are prepared through deposition. The gallium oxide X-ray detector of the present invention can work at room temperature, can also withstand high voltage and work in harsh environments.