The invention provides a THz single photon detector based on graphene quantum dots and a preparation method thereof. A basic structure with graphene single-quantum dots or the mutual coupling of graphene dual-quantum dots in a series connection and a nanoribbon graphene electrometer is employed, with a silicon substrate, a source electrode, a drain electrode, a side gate, a back gate, a Coulomb island, an electrometer and a protective layer as basic composition units, the source electrode, the drain electrode, the side gate, the Coulomb island and the electrometer are arranged in an integrated way on a silicon dioxide base formed at the surface of the silicon substrate, and the back gate is arranged on the silicon base of the silicon substrate, wherein the graphene single-quantum dots or graphene dual-quantum dots in a series connection are taken as the Coulomb island of the THz single photon detector, the Coulomb island is between the source and drain electrodes and the side gate, and a graphene nanoribbon is integrated near the Coulomb island as the graphene electrometer. The Coulomb island and the source and drain electrodes are coupled in the form of a tunnel junction, and the Coulomb island, the side gate, the back gate and the graphene electrometer are coupled in the form of capacitance. According to the THz single photon detector and the preparation method, the problems of large technical difficulty and low sensitivity in a present preparation method are overcome.