The present invention discloses a solar-blind AlGaN ultraviolet (UV) photodetector and a preparation method thereof. The solar-blind AlGaN UV photodetector comprises an UV photodetector epitaxial wafer, including an undoped N-polar plane AlN buffer layer, a carbon-doped N-polar plane AlN layer, a carbon-doped N-polar plane composition-graded Al.sub.yGa.sub.1-yN layer, and an undoped N-polar plane Al.sub.xGa.sub.1-xN layer that are grown sequentially on a silicon substrate, and also comprises an insulating layer, an ohmic contact electrode, and a Schottky contact electrode arranged on the UV photodetector epitaxial wafer, as well as a SiN.sub.z passivation layer arranged on both sides of the UV photodetector epitaxial wafer, where x=0.5-0.8, y=0.75-0.95, and z=1.33-1.5. The present invention realizes the preparation of the high-performance solar-blind AlGaN UV photodetector, and improves the responsivity and detectivity of the AlGaN UV photodetector' in the UV solar-blind band.