As a new type of transmission, GIL is widely used all over the world. Compared with gas insulated switchgear (GIS), GIL has a greater possibility of internal insulating gas leakage. In UHV GIL, the total amount of SF₆ gas is up to about 750 tons. A small amount of gas leakage may cause power system fault, environmental pollution and even personal injury and death, It is necessary to improve the leakage rate index greatly and strictly control the gas leakage. In view of the requirement for extremely low gas leakage rate of UHV GIL, this paper proposes the method and key parameters of helium mass spectrometry vacuum leak detection. A helium mass spectrometry leak detection system with a length of 20 meters is applicated for the first time in the world. Through the research and comparison of the leak detection methods, two common air-tightness detection methods (local binding cover method and buckle cover method) were analyzed by experiment. Considering the detection efficiency and detection accuracy, a high-precision, quantifiable helium leak detection method combined with the SF₆ partial buckle cover and fixed volume buckle cover is proposed in the type test and the factory test. The results show that SF₆ annual leakage rate in GIL compartment is decreased from 0.5% to 0.01% and the minimum leak detection rate of the system reaches 1×10^-8Pa m3/s. This study bcan provide a basis for further improving the gas leak detection efficiency in the future.