A graphene / 4H-SiC alpha particledetector and a preparation method thereof, and relate to the technical field of semiconductorradiation detectors. The detector comprises: a 4H-SiC substrate, and an n-type epitaxial layer located on the upper surface of the 4H-SiC substrate; an ohmic contactelectrode formed on the lower surface of the 4H-SiC substrate; and a Schottky incident window electrode formed on the upper surface of the n-type epitaxial layer; wherein the Schottky incident window electrode is composed of graphene with an atomic level thickness, and the graphene forms a Schottky contact with the n-type epitaxial layer. The atomic level thickness of graphene is used to suppress the "dead layer" effect of the incident window, thereby improving the energy resolution of the detector.