Aiming at the requirement of highly sensitive perception of gas insulated switchgear (GIS) intermittent partial discharge leakage electromagnetic wave. In this paper, the rectangular waveguide theory is used to analyze the leakage electromagnetic wave propagation characteristics and energy concentration frequency band of GIS rectangular pouring hole combined with finite element simulation, and an external GIS partial discharge detection back-cavity butterfly antenna sensor is designed by combining the design principle of butterfly antenna and trapezoidal gradient balun. In the 1 GHz～3 GHz frequency band, the maximum voltage standing wave ratio (VSWR) of the sensor is less than 7, the minimum VSWR is 1.1, and the average VSWR is 2.9; in the 0.3 GHz～1 GHz frequency band, the maximum and minimum VSWR reach 42.8 and 7, respectively, and the average standing wave ratio can reach 20.6. According to the design results, the antenna sensor was trial-produced and tested with intermittent partial discharge of typical metal protrusions. The test results show that the sensor designed in this paper can stably sense the partial discharge electromagnetic wave signal leakage of the pouring hole, and the electromagnetic wave signal energy concentration frequency band is greater than 1 GHz, which is consistent with the theoretical analysis of the leakage electromagnetic wave of the pouring hole and the VSWR performance of the sensor design.