Abstract: During the lightning impulse test on the gas-filled cabinet, the top expansion bushing experienced a breakdown. This paper dissects the broken bushing and analyzes the electric field distribution using electric field simulation software, revealing that the original bushing had insufficient insulation margin. Based on this, a new type of bushing with a larger insulation margin is designed. Simulation verification shows that the electric field distribution of the new bushing has been significantly improved, and its insulation performance has been enhanced, effectively solving the breakdown issue of the top expansion bushing during lightning impulse tests and actual operation.