Free moving metallic particles are one of the most common defects leading to insulation failures in gas insulated substation (GIS) equipment. The present research is designed to diagnose and assess the severity levels of partial discharge (PD) initiated by free moving metallic particles on GIS insulator surface. On a well-established 252 kV GIS experiment platform, long term tests were conducted to observe the entire development process of partial discharge from its very initiation stage till eventual flashover as the applied voltage gradually boosted. Real time measurement was undertaken during the tests to capture the trend curve of partial discharge as a result of test time, scatter plot, histogram, grey-scale map, and time-frequency map of partial discharge. The experimental results indicate that 1) partial discharge provoked by free moving metallic particles in GIS goes through three severity levels, namely, minor discharge, moderate discharge, and threatening discharge; 2) as partial discharge develops, the phase distribution is widening and corresponding shape changes occur in the scatter plot, histogram, grey-scale map, and time-frequency map. It is recommended that the features of phase distribution and the corresponding spectra could be used as important criteria in diagnosing and assessing the severity levels of partial discharge initiated by free moving metallic particles on GIS insulator surface