At present, GIS equipment has become the core equipment of power system. At present, the frequent insulation faults of GIS equipment seriously threaten the safe operation of power system. At present, the UHF detection method as a commonly used GIS partial discharge detection method is difficult to effectively warn the GIS insulation defects. In order to improve the effectiveness of the insulation defect early warning of the ultra-high frequency detection method, this paper selects the actual case of typical insulation defects for investigation, analyzes the development process of partial discharge before flashover of typical defect faults, and provides on-site basis for the improvement of the effectiveness of the early warning ability of the subsequent ultra-high frequency detection method, which has important research significance.