Gas insulated switchgear often works under high current and strong electrical power and for long periods. In addition, repeated operation of the switchgear with long-term mechanical vibration of the equipment easily leads to the loosening of the fasteners at the electrical connections. This contributes to GIS abnormal vibration, or even air leakage and insulation failure. In this paper, a vibration detection method is adopted for the loosening failure of the GIS bus conductor. First, theoretical research on the vibration of the normal and loose fault is carried out. Then, a 126 kV vibration detection experimental platform is built to study the characteristics of the vibration signal when the bus conductor is loose. The results indicate that the vibration frequency of the GIS bus bar shell is within 1 000 Hz, and 100 Hz is the main mechanical vibration frequency if the GIS works well. When a bus conductor failure occurs, the frequency component above 1 000 Hz increases significantly in proportion to the severity of the failure. Therefore, the frequency domain energy which is significantly increased between 1 000~3 000 Hz and 0~1000 Hz can be defined as E1~3 kHz and E0~1 kHz and used as characteristic parameters. Finally, the ratio of E1~3 kHz and E0~1 kHz is defined as the loosening fault coefficient γ to reflect the severity of the loosening fault. Calculations show that when γ＞0.04, the bus conductor loosening fault occurs, and the larger the value, the more serious the fault. If γ≥0.1, a serious loosening failure may occur. This work is supported by the National Nature Science Foundation of China (No. 51577050) and the Key Science and Technology Project of State Grid Tianjin Electric Power Company (No. KJ19-1-19).