In response to growing environmental concerns, this study proposes the use of dry air as an insulating gas in gas-insulated switchgear (GIS) to replace the traditional SF6 gas. To address the problem of gas leakage, the helium tracer method is employed to detect the leakage of insulating gas within the equipment, focusing on the impact of helium addition on the insulation performance of GIS. Electrodes and experimental chambers were designed to simulate the electric field conditions under actual operating circumstances. Breakdown voltage experiments at power frequency were conducted to study the insulation performance of mixed gases with three different concentrations of helium under two pressure conditions. The breakdown voltage values at power frequency were theoretically analyzed from the perspective of gas discharge. Additionally, the breakdown voltage dispersion of mixed gases with different concentrations of helium was analyzed, considering the operational stability of GIS equipment. The experimental results indicate that the addition of helium has a negligible effect on the insulation performance of dry air when compared to the pure dry air power frequency breakdown results. This confirms that the helium tracer method is an effective technique for detecting the leakage of insulating gas in environmentally friendly dry air GIS equipment.