Gas insulated switchgears（GIS） occupy an important position in the power system. Discharge on insulating spacer surface is an important factor influencing the safe operation of GIS. The authors set up different insulation defect models on insulating spacer surface and use UV camera to observe the surface discharge phenomena. Conclusions are drawn that: 1 The ultraviolet photon number increases with the increasing length of the insulation defect models

however

saturation characteristic is observed in repetitive experiments; 2 The initial discharge voltage ant the photon number under certain voltage significantly depend on the position of an insulation defect; 3 When the distance between an insulation defect and the guide bar is more than 6 cm

the discharge weakens obviously under the same voltage

and the corresponding ultraviolet photon number reduces significantly. These conclusions are helpful to the further study on the characteristics of insulator surface charge

and to the on-site detection of GIS partial discharge.