The　majority　of　failures　of　electrical　systems　are　due　to　excess　electrical　resistance,　erosion　and　welding　of　the　contacts.　Especially,　the　arc　between　the　contacts　during　switching　the　load　makes　the　material　on　the　surface　of　the　contact　phase　change,　such　as　melting,　vaporizing　etc.,　which　may　be　fatal　to　the　contacts.　The　finite　element　software　is　used　in　this　paper　to　calculate　the　phase　change　under　the　arc.　The　latent　heat　absorbed　or　released　during　the　phase　change　process　and　the　material　performance　dependence　upon　the　temperature　are　taken　into　account　when　the　temperature　distribution　on　the　whole　contact　is　calculated.　The　position　that　reaches　the　phase　change　temperature　is　acted　as　the　interface　of　phase　change,　and　then　the　range　of　erosion　is　determined.　Finally,　experiment　has　been　done　to　validate　the　analysis　result.　It　shows　that　the　erosion　area　by　calculation　is　similar　to　the　experimental　result.