In addition to the resulting field current increment and reactive power output decrement， the inter-turn short circuit of field windings in large hydro-generators will also introduce the aggravation of vibration， which jointly affect the secure and stable operation of power systems. In order to study the fault characteristics and mechanisms， the Three Gorges left bank VGS generator is used as example for this study. Firstly the properties of the field MMF and the uneven air-gap field generated by the MMF under fault conditions are studied. It can be concluded that there exist fractional harmonic unbalanced branch current inside each phases of the stator. Then the stator branch can be used as the basic analysis module， the relationship between time harmonic frequency of branch current and space harmonic frequency of the armature reaction MMF can be derived; and the routine general formula of the harmonic frequency of the additional current in the rotor windings produced by the armature reaction field can be obtained as well. Finally faults can be calculated quantitatively using a multi-loop method as verified by experiments， which prove the validity of the theoretical analysis. Based on the calculation results， protection of the faults is also preliminarily discussed. This paper initially puts forward the analysis method of the inter-turn short circuit fault characteristics of field windings in fractional-slot hydro-generators. It forms the theoretical foundation for the fault on-line detection and protection for fractional-slot hydro generators.