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High-voltage electrode protrusion is one of the major causes of insulation failure in gas-insulated substation (GIS) equipment. The present research is designed to diagnose and assess the severity levels of the partial discharge (PD )provoked by high-voltage electrode protrusion on the GIS insulator surface. With the application of boosting voltage approach, long-term tests were conducted on a well-established 252-kV GIS experiment platform to observe the entire developing process of PD from its very initiation until the eventual flashover. Real-time measurement was also undertaken during the tests to capture the trend curves and the spectra of the PD. The study results indicate that 1) PD initiated by high-voltage electrode protrusion on GIS insulator surface undergoes through three severity levels, namely, petty discharge, medium discharge, and threatening discharge, and 2) as PD exacerbates, the phase distribution keeps widening and corresponding shape changes are shown in the scatter plot, the histogram, the gray-scale map, and the time-frequency map. It is suggested that the features of phase distribution and the corresponding spectra be used as important referential criterion in diagnosing and assessing the severity levels of the PD triggered by high-voltage electrode protrusion on the GIS insulator surface. [ABSTRACT FROM AUTHOR]