The rotor of large hydro-generator, including the pump storage generator, is an explicit-pole rotor with multi-turn and long winding. The traditional fault diagnosing method of winding inter-turn short circuit based on AC impedance measurement has shortcomings in sensitivity and convenience. Based on the theory of multi-conductor transmission line, the port frequency domain characteristic model of the explicit-pole rotor winding is established, and the fault diagnosing and locating methods are proposed through the simulation analysis. Finally, the validity of the diagnosis method is verified by the fault simulation and test of the real rotor. The simulation and test results show that the proposed method can realize the diagnosis and location of winding inter-turn short circuit fault without disconnecting the electrical connection between magnetic poles.