In order to use current transformer installed for the generator main protection to monitor inter-turn short circuits of field winding， taking Three Gorges VGS generator as an example the after-fault unbalanced current entering into various main protections is calculated and analyzed with multi-loop method. Then the defects of fault monitoring using single harmonic in unbalanced current are pointed out and a monitoring principle of field winding inter-turn short circuit based on unbalanced current effective value for the main protection is proposed. Through the calculation and theoretical analysis of faults with different short circuit turn ratios of field winding， it is pointed out that unbalanced current flowing between two neutral points is not suitable for reflecting inter-turn short circuits fault of field winding， and the monitoring method reflecting the internal unbalanced current of each phase winding should be adopted instead. Finally the coordination between the inter-turn short circuits monitoring of field windings and other conventional protections is illustrated， laying a theoretical foundation for the field windings inter-turn short circuit fault on-line monitoring.