Abstract: The Sutong GIL utility tunnel project represents the world's most advanced ultra-high-voltage GIL innovation, featuring the highest voltage level, largest transmission capacity, and most sophisticated technical solutions. Utilizing Gas-insulated metal-enclosed transmission Line (GIL) technology, this project enables river crossing power transmission through a tunnel beneath the Yangtze River. The tunnel spans 5468.5 meters in length, with a maximum burial depth of -74.83m and a peak water pressure of 0.8MPa. It overcame global technical challenges, including high-reliability insulation design, ultra-long-distance GIL installation and testing, and tunnel construction under extreme hydrostatic pressure. Through independent R&D of ultra-high-voltage GIL equipment, the project achieved homogenized multi-physical field design for insulators, precise sealing control of SF6 gas, and millimeter-level installation accuracy, ensuring highly reliable operation. This project completed the east China ultra-high voltage AC double-circuit loop network, significantly enhancing the grid's transmission capacity and security. It provides a Chinese solution for river-crossing and subsea power transmission, offering substantial technical, economic, social, and environmental benefits.