To control the greenhouse effect and reduce the use of SF₆ gas, the gas insulated switchgear (GIS) is gradually trying to use environmentally friendly gas and clean air as the insulation medium, but the gas pressure must meet the rated pressure. In order to detect leakage in time, tracer helium is added into clean air to determine the leakage state of insulating medium and the location distribution of leakage points by monitoring the tracer helium. Combined with the clean air leakage detection and evaluation experimental device model, SpaceClaim is used to build a two-dimensional gas transfer phenomenon model based on FLUENT software, and the leakage diffusion distribution law after filling different pressure tracer helium in the clean air background in the GIS bus with actual size is simulated. After 1kPa of tracer helium is filled, the distribution of tracer helium at different leakage points and the time sequence of detection of helium at different leakage points are analyzed. The results show that the helium concentration near the inner wall of the GIS bus is higher than that in the interlayer area. When the partial pressure of helium is 1kPa, the presence of tracer helium is detected at positions A, B and C of the leakage port in the 50s, 350s and 850s, respectively. The experimental results of sensor detection of tracer helium leak show that the detection time of tracer helium at the three leakage ports A, B and C are 39s, 313s and 809s respectively, which is basically consistent with the theoretical analysis results based on FLUENT. The gas concentration detected at the three leakage ports after diffusion equilibrium in the bus is 1 402µL/L, which is in good agreement with the theoretical tracer helium concentration of 1 426µL/L prepared by partial pressure method. This method can provide theoretical reference for clean air leakage detection in GIS and the optimal location of leakage point sensor.