Abstract: Since the GIL lines are filled with SF6 gas, there is a possibility of leakage accident of SF6 gas in GIL utility tunnel due to manufacturing, transportation, installation and aging, so the corresponding monitoring,control system and effective exhaust system should be established. Based on the 6.3 km super-long GIL cabin of the underground utility tunnel at Wuhan Tanxinpei road, the most unfavorable leakage position and working condition after gas leakage are determined considering the data of heat-exhaust ventilation and SF6 gas diffusion characteristics.With the open source computational fluid dynamics software OpenFOAM,a 3D long-segment model and a detailed short-segment model for SF6 gas diffusion are set up, and the numerical analysis process is presented. The boundary condition is based on the data extracted from heat-exhaust ventilation analysis, the typical accident conditions of SF6 gas are analyzed and the variation trend of SF6 gas concentration field in GIL cabin is obtained. The analysis results show that SF6 gas is mainly discharged by the conventional exhaust ventilation system and the auxiliary system can largely save the exhaust time. The numerical simulation method can provide reference for the simulation of other types of heavy gas leakage and diffusion in the pipe gallery.