To address the challenges posed by the complex distribution of sulfur hexafluoride (SF6) gas leakage in gas-insulated transmission line (GIL) utility tunnels, this paper proposes a simulation-based risk assessment approach based on digital twin technology. Firstly, a general exclusion plan for large-scale gas leakage is summarized based on the actual working conditions of the Su-Tong GIL utility tunnel and the deployment of the ventilation system. Then, based on the physical process and mathematical mechanism of SF6 gas leakage, a digital twin model of the utility tunnel is constructed in virtual space, and the constraints of gas leakage are set. Secondly, by simulating the diffusion distribution of large-scale SF6 gas leakage in the twin space, the distribution data of SF6 during the exclusion process is deduced and visualized. Finally, combining the visualization results of each stage, a full-process risk assessment of gas leakage in the GIL utility tunnel is carried out, providing a reference for formulating safe evacuation plans and achieving intelligent monitoring in the corridor.