In this paper, a novel frequency index for Inter Turn Short Circuit (ITSC) fault diagnosis of the Wound Field Synchronous Generator (WFSG) is presented by utilizing the air gap flux density and the total force density. The analytical expression of the force density is derived to detect the sideband harmonics of the vibration in the faulty WFSG by considering the impacts of rotor saliency, saturation, stator slots, and ITSC fault. The 2-D finite element method is employed to extract the characteristics of the faulty machine. The harmonic force response of the healthy and faulty WFSG acquired from the magnetic analysis is used as an input to the mechanical analysis to get the deformation, modal analysis of faulty WFSG.