In order to find a new method to the online identification of inter turn shortcircuit fault of brushless excitation synchronous generator rotor, the change of electromagnetic properties was analysed and electrical parameters after the synchronous generator rotor windings occurred inter turn shortcircuit. According to the characteristic that the exciting current was increasing and the reactive power output was decreasing relatively, a method was put forward which uses generator reactive power output relative change rate which was calculated or measured before and after the fault as a practical criterion for identification of the severity of brushless excitation synchronous generator rotor windings inter turn shortcircuit fault. Finally, the relative experiment was carried out using the dynamic analog experiment unit.