This paper develops an online inter-turn short-circuit fault (ISF) detection method for inverter-fed permanent magnet synchronous machine (PMSM) systems. A high-frequency (HF) mathematical model of a PMSM with an ISF is established. Based on the presented HF model, the HF responses of the PMSM with various severity of ISF under HF voltage injectionare analyzed in detail, where both the short-circuit ratio and the fault resistance are considered to evaluate the fault severity. In the proposed fault diagnosis method, the amplitude of the HF negative-sequence component is used to determine the fault severity, and the initial phase is used to locate the position of the faulty phase. The simulation and experimental results verify the effectiveness of the proposed method.