The　trend　of　development　of　Gas　Insulated　System　(GIS)　recently　includes:　miniaturization,　intelligentization　and　high　reliability.　This　needs　a　higher　insulation　performance　of　GIS.　Free　conducting　particle　in　GIS　is　a　main　reason　that　causes　the　degradation　of　GIS　insulation.　Free　conducting　particle　will　bring　a　high　risk　to　GIS　insulation　when　it　approaches　to　the　high　voltage　conductor　or　reaches　the　disc-type　insulator　surface.　Particle　in　radial　non-uniform　electrical　field　is　apt　to　move　along　the　radial　direction　in　GIS,　and　probably　moves　into　the　high　insulation　risk　region.　According　to　the　distribution　of　the　electrical　field　between　coaxial　electrode　systems,　the　dynamic　model　of　spherical　particle　in　axial　non-uniform　electrical　field　under　AC　voltage　is　amended.　The　motion　phenomenon　of　particle　is　explained　by　solving　the　dynamic　equations.　Gravitational　force,　Coulomb　force,　electrical　gradient　force　and　gas　viscous　force　are　considered.　Improving　the　applied　voltage　can　increase　the　motion　distance　in　vertical　direction.　The　criterion　can　explain　the　incremental　quantity　of　the　motion　onset　voltage　of　particle　in　uniform　field　under　AC　voltage　than　it　under　DC　voltage.　Observations　indicate　that　voltage　exerts　an　incremental　effect　on　maximum　height.　By　analyzing　improved　dynamic　model　of　spherical　particle,　another　novel　criterion　to　judge　the　penetration　of　the　motion　of　spherical　particle　in　uniform　field　under　AC　voltage　is　brought　forward,　as　well.