Mechanical defect is one of the main reasons leading to the failure of gas insulated metal enclosed switch（GIS）equipment. However

the site detection devices and mechanical defect assessment methods are not available on GIS mechanical fault diagnosis technology. Consequently

we first studied the vibration mechanism and diagnosis method of GIS

and developed a GIS vibration signal detection device after adopting the vibration acceleration sensor as a sensor. On this basis

a mechanical vibration diagnostic platform of 550 kV GIS was established in laboratory

a circuit resistance tester was adopted to quantify the contact state of disconnector

and the vibration signal in GIS was detected based on different abnormal contact conditions in GIS disconnector. Finally

a method for extracting the characteristic quantity of abnormal vibration signal and an assessment method were studied. The results indicate that the vibration signal of 100 Hz can be detected and the amplitude in horizontal direction is slightly higher than that in vertical direction when the GIS disconnector operates well. However

obvious vibration signals can be detected in the frequency of 50 Hz and 200 Hz in frequency domain spectrum（FDS） when the GIS disconnector has a loose contact. The amplitude of 100 Hz can be taken as the reference value

while the ratio of amplitudes between 50 Hz and 100 Hz and the ratio of amplitudes between 200 Hz and 100 Hz can be regarded as the characteristic parameters to characterize the connection state of disconnector. According to experiment data

if k200>0.2 and 0.1<k50<0.3

it can be convinced that the GIS disconnector has a loose contact.If k200>0.5 and k50>0.2

it can be convinced that there is serious a mechanical defect in the GIS disconnector.