In　order　to　improve　the　diagnose　ability　of　insulation　defects　of　gas　insulated　switchgear　(GIS)　and　improve　the　detection　experience　of　ultra-high　frequency　(UHF)　method　in　the　field,　detailed　tests　and　diagnosis　of　multiple　voids　discharge　of　550　kV　GIS　insulation　spacer　were　performed　in　this　paper.　Firstly,　the　abnormal　signal　of　550　kV　GIS　insulation　spacer,　presenting　a　characteristic　of　void　discharge,　was　detected　on　field　by　the　UHF　method.　After　the　replacement　of　the　insulation　spacer,　the　tests　of　withstand　voltage,　partial　discharge,　lightning　impulse　and　X-ray　detection　were　employed　to　investigate　the　void　discharge　of　insulation　spacer.　Meanwhile,　the　UHF　and　pulse　current　methods　were　used　to　detect　the　PD　signal　of　the　insulation　spacer.　The　discharge　characteristics　were　compared　after　the　lightning　impulse.　Finally,　further　diagnostic　analyses　were　carried　out　by　using　time　difference　location　statistics　and　multi-source　discharge　separation　methods.　The　results　show　that　there　are　8　voids　in　the　insulation　spacer,　which　are　linearly　distributed.　The　amplitudes　of　the　signals　are　positively　correlated　with　UHF　and　pulse　current　method.　With　the　increment　of　applied　voltage　amplitude,　the　discharge　spectrum　gradually　changes　from　single　void　discharge　to　multiple　voids　discharge.　The　partial　discharge　initial　voltage　decreases　and　the　extinguishing　voltage　does　not　change　after　the　lightning　impulse.　The　UHF　signal　amplitude　and　discharge　amount　will　increase　under　the　same　voltage　after　the　lightning　impulse.　The　time　difference　location　statistics　and　multi-source　separation　indicate　the　insulation　spacer　behaves　as　multiple　discharge　source　characteristics,　which　is　consistent　with　the　X-ray　results.　The　results　can　provide　field　experience　for　GIS　UHF　detection　and　diagnosis.　©　2020,　High　Voltage　Engineering　Editorial　Department　of　CEPRI.　All　right　reserved.