Abstract: In response to the urgent demand for high-capacity, lightweight, and high-reliability gas-insulated metal-enclosed switchgear (GIS) at the 110kV voltage level in China"s 500kV terminal substations, this paper systematically conducts research and tackles key technical challenges related to three-phase, common-tank GIS with a rated voltage of 126kV, a rated current of 5000A, and a rated short-circuit breaking current of 50kA. Through a series of core technological breakthroughs, including coupled electric field-temperature field optimization design, research and development of high-current-carrying and efficient heat dissipation structures, precise matching of self-energy arc-extinguishing system characteristics, and mechanical reliability multi-body dynamics simulation and verification, a lightweight GIS prototype has been successfully developed, and type testing has been completed in accordance with relevant national standards. This equipment can significantly reduce the footprint of substations, optimize the regional power grid structure, and enhance the reliability and stability of power supply in urban load centers. It provides a novel technical solution for the construction of 500kV terminal stations in high-density load areas and holds significant engineering application value and theoretical research significance.