The invention discloses a system and a method of diagnosing GIS (Gas Insulated Switchgear) mechanical defects based on abnormal vibration analysis. An acceleration sensor is fixed at the position of a measurement point on the surface of the GIS shell, and the acceleration sensor, a charge amplifier, a data acquisition unit and a data processing device are serially connected in sequence; after the data processing device carries out wavelet denoising on measured vibration signals, a spectrum for vibration signals at each measurement point is obtained finally; and through analyzing the signal component of each frequency range of the spectrum, the GIS mechanical fault type is determined. The system and the method of the invention have the beneficial effects that the GIS diagnosis technology research based on abnormal vibration analysis provides an important and feasible detection means for discovering GIS mechanical defects or hidden fault, and an important role is played in ensuring reliable operation of the GIS.