The looseness fault of switch parts and electrical connection points often occurs in the operation of gas insulated switchgear (GIS), which leads to abnormal mechanical vibration. The vibration mechanism of three-phase community GIS and three-phase split GIS is derived from the electromagnetic force and eddy current on the conductive rod respectively, and the correctness of the vibration mechanism is proved by simulation modeling. The mechanical vibration model of the contact surface in the case of looseness fault is established, and the vibration displacement of the contact surface in the case of looseness is calculated by simulation. By comparing the vibration characteristics of normal operation with those of looseness fault, it is found that the energy of vibration signal will increase in some frequency bands when loosening fault occurs, and the energy increase is obvious in the 1 000~2 000 Hz range. By simulating the actual looseness fault on site, collecting the GIS vibration signal, and extracting the spectrum characteristics of the vibration signal by time-frequency transformation, the correctness of the simulation calculation is verified.