Abstract: In recent years, the mechanical defects of the EHV/UHV GIL equipment occur frequently. It is the key to master the natural vibration modes and the abnormal vibration characteristics of such equipment in order to effectively detect the mechanical defects. In this paper, based on a 550 kV GIL high current excitation vibration simulation platform, the vibration characteristics under the normal condition and the abnormal one of having a loose bus base are compared and analyzed. The modal characteristics, the vibration signal propagation characteristics, and the effects of the current and the bus length on the mechanical vibration behavior of the equipment are discussed. The results show that the natural modal vibration frequency of the GIL equipment is mainly concentrated in between the range of 800Hz-2700 Hz. When the bus base loosens, the vibration frequency response function curve of the GIL equipment changes significantly, and the natural vibration frequency and the damping ratio decrease with the vibration of the guide rod strongest in the radial direction. When the load current is in between 3000-5000 A, the nonlinear vibration of the equipment is enhanced by such high current excitation, more helpful to detect the mechanical defects. Compared with the situation under the normal conditions, the vibration acceleration value and the vibration odd-even response ratio of the GIL equipment under the loose bus base mechanical defect increases significantly. Moreover, in the frequency bands of between 1200-1800 Hz and between 2200-2600 Hz, significant ultra-harmonic response phenomena related to the inherent modes are generated. This feature provides a valuable reference for judging the loose bus base mechanical defect of the GIL equipment.