The invention relates to a terahertz single-photon detector and a detection method thereof. The terahertz single-photon detector comprises a semiconductor heterojunction, a detection electrode formed on the surface of the semiconductor heterojunction, a first quantum dot and a single-electronic gauge formed on the surface of the semiconductor heterojunction, wherein two-dimensional electron gas is formed on the surface of the semiconductor heterojunction; the detection electrode is used for collecting terahertz single photons; the terahertz single photons collected by the detection electrode are used for triggering the first quantum dot to carry unit positive charges; the single-electronic gauge is used for measuring the charge change of a static environment at the periphery of the single-electronic gauge, and is in capacitive coupling with the first quantum dot; and current in the single-electronic gauge is hopped by the unit positive charges carried by the first quantum dot. The terahertz single-photon detector can be used for detecting weak terahertz radiation and counting the terahertz single photons.