Abstract: Ultra-high frequency(UHF) method is commonly used to detect partial discharge(PD) insulation defects of transformers, gas insulated switchgear(GIS), etc.. It is very important to ensure that UHF antenna has good PD high-frequency electromagnetic signal sensing performance under the limited built-in installation space. In this paper, a miniaturized UHF monopole antenna is designed based on polyimide(PI) flexible base. Its size is only 58 mm×36.8 mm×0.3 mm, which is 50.4% smaller than that of the traditional monopole antenna, and its low-frequency performance is improved by hollowing out and bending the radiation area. Simulation and measurement results show that the effective bandwidth of the flexible antenna in insulating oil and SF₆ gas can reach 360 MHz~2.35 GHz and 440 MHz~1.78 GHz, respectively. It can receive PD UHF signals omnidirectionally and keep good radiation performance after bending. Finally, a 220 kV real GIS and transformer PD simulation test platform is built to verify the PD detection performance of the antenna under different insulation defects. The results show that the built-in flexible antenna can effectively detect PD UHF signals under low discharge magnitude, which has high sensitivity and meet the PD detection demands of power equipment.