Abstract: The surface partial discharge triggered by immobilized metal particles on gas insulated switchgear（GIS） insulators in terms of the discharge phenomena alongside its evaluation process was studied in this paper.With application of incremental voltages,long-term laboratory tests were conducted on a well-established 220 kV experimental platform to observe the entire evaluation process of surface partial discharge from its initiation,to gradual development and the eventual occurrence of surface flashover.Features of surface partial discharge phenomena at various evolutionary stages were recorded in the form of frequency curves,scatter diagrams,two-dimensional histograms,and grey-scale maps.It was shown by experiments that the phenomena of the surface partial discharge triggered by immobilized metal particles on GIS insulators indicates three developing stages from corona discharge domination,to coexistence of corona discharge and surface streamer discharge,and finally,surface streamer discharge domination.