Vibration harmonics with an exponential decay are observed by distributed acoustic sensing in a gas-insulated met-al-enclosed transmission line. The vibration is attributed to the periodic Lorentz force induced by current oscillation, leading to harmonics from 50 Hz to 350 Hz with equal spacing about 50 Hz. Moreover, the harmonics with an expo-nential decay in intensity are recognized along with their orders, which agrees with prediction of a damped structural response at R2 of 0.992. Our demonstration promises practicality in vibration monitoring and risk assessment in the high-risk GIL operation and maintenance.