Gas-insulated Transmission Line（GIL） is very suitable for the power transmission across the Changjiang River in Huainan-Nanjing-Shanghai 1000 kV UHV project because of its high reliability and long endurance.Because the size of GIL unit in this project are 18 m in length and 900 mm in diameter

the probability of leakage is higher than GIS whose leakage rate is≤0.5% per year in standard

and the leakage rate is prescribed as≤0.01%per year in this project in order to enhance the dielectric strength and to avoid the external influence.The object of this paper is to obtain an air tightness method that is useful in type test and routine test

and to determine the duration in keeping Helium pressure for leakage detection that is important for accuracy and efficiency in routine test.In the test

two common methods of air tightness detection are analyzed

and the air tightness detection scheme of helium leak detection of vacuum box is proposed to meet the requirements of the project.In addition

the duration in keeping Helium pressure for leakage detection of GIL is studied with standard leakage unit and 18 m GIL unit

respectively.The experimental results indicate that Helium leakage detection method is enough for leakage detection with the required accuracy and efficiency.Moreover

the duration in keeping Helium pressure is useful for employing Helium leakage detection in the routine test of UHV GIL.