Ultra-high voltage gas insulated combined electrical switchgear is a key core equipment to ensure the safe operation of the ultra-high voltage power grid and quickly remove system failures. However, operating statistics show that discharge failures induced by metal particles in GIS account for as much as 54.5%. Therefore, how to effectively avoid the fault caused by UHV GIS inducing discharge of metal particles has become the primary issue in the operation and maintenance of UHV GIS. In response to this problem, this paper has built a true experimental platform for ultra-high voltage GIS, aiming to study the variation pattern of local discharge of pulse current before flashover of metal particles on the surface of ultra-high voltage GIS basin insulators. Experimental results show that flashover is prone to occur when there are 8 mm long metal particles in the high field strength area of the basin insulator surface. Through the above analysis, this paper not only reveals the potential risks of metal particles induced discharge on the surface of UHV GIS basin insulators, but also provides experimental basis and reference data for subsequent research.