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A major cause of insulation failure in GIS equipments is due to the high-voltage electrode defect, which usually occurs at the interface formed by metallic conductor, insulator and SF6. The present research is designed to diagnose and assess the severity levels of the partial discharge (PD) provoked by high-voltage electrode defect on GIS insulator surface. With the application of gradually increased voltage, long term tests were conducted on a well-established 252 kV GIS experiment platform to observe the entire evolution process of partial discharge from its very initiation till the eventual flashover. Real time measurement was also undertaken during the tests to capture the trend curve of partial discharge as a result of test time, the scatter plot, histogram, grey-scale map, and the time-frequency map of the partial discharge. The test results indicate that 1) partial discharge initiated by highvoltage electrode defects undergoes three elocutionary phases, namely, the inception phase, the developing phase and the threatening phase; 2) as partial discharge exacerbates, the phase distribution is widening and corresponding shape changes occur in the scatter plot, histogram, grey-scale map, the timefrequency map and the ultraviolet images. It is suggested that the features of phase distribution and the corresponding spectrums be used as important criterion in diagnosing and assessing the severity levels of the partial discharge triggered by high-voltage electrode defect on GIS insulator surface.