In this paper, the electromagnetic characteristics on rotor windings inter turn short-circuit of steam generator has been analyzed and calculated. The phenomenon, which the variation of main magnetic field of rotor windings is more apparent than the stray magnetic field when the rotor winding inter turn short-circuit happens, has been found. Using the stator winding as the fault detection winding, the corresponding relationships between the faults levels of rotor winding inter turn short-circuit and the effective magnetic field loss is established. The relevant verification has been carried using the dynamic simulative experimental units, and the result coincides with the theory deduction.