The　standard　lightning　impulse　(LI),　defined　as　a　smooth　full　lightning　impulse　voltage　having　a　front　time　of　1.2　mu　s　and　a　time　to　half-value　of　50　mu　s,　obtained　from　field　observation,　has　remained　unchanged　from　IEC　Ed.　1　in　1962　through　to　Ed.　3　in　2010.　LI　test　plays　an　important　role　in　the　equipment　design　and　insulation　defects　detection.　In　order　to　evaluate　the　reasonability　of　the　standard　LI　waveform　from　the　point　of　insulation　characteristics,　breakdown　characteristics　of　GIS　under　LI　with　different　wavefront　times　T-f　and　wavetail　times　T-t　were　studied.　Those　experimental　results　show　that　the　LI　waveform　parameters　have　little　influence　on　the　breakdown　characteristics　of　a　sound　GIS　equipment,　but　have　great　influence　on　the　GIS　equipment　with　defects.　For　a　highly　inhomogeneous　electric　field,　with　the　wavefront　time　increasing,　the　50%　breakdown　voltages　U-50%　have　remarkable　change　and　the　U-50%　-　T-f　curve　tends　to　be　U-shaped.　The　50%　breakdown　voltages　decrease　significantly　with　the　increase　of　impulse　wavetail　time,　especially　when　wavetail　time　is　in　the　range　of　1.5　to　15　mu　s,　but　the　difference　of　the　values　decreases　after　15　mu　s.　Ignoring　the　generating　possibility　in　the　field　for　the　large　series　inductance　and　load　capacitance,　a　LI　with　wavefront　time　in　the　range　of　0.2　to　0.7　mu　s　is　proposed　in　the　field　for　it　may　increase　the　probability　of　detecting　insulation　defects.　Finally,　the　polarity　reversion　phenomenon　and　critical　radius　phenomenon,　observed　from　the　breakdown　characteristics,　can　be　explained　by　the　migration　and　diffusion　of　space　charges.