With　the　development　of　HVDC　transmission　technology,　HVDC　converter　valves　are　widely　used　in　many　areas　including　the　connection　between　systems　with　different　frequencies.　Thyristor　is　the　core　component　in　HVDC　converter　valve　and　its　reliability　will　influence　the　operation　of　HVDC　converter　valve.　However,　the　structure　of　thyristor　is　fragile　under　over-voltage　condition.　On　the　one　hand,　external　shock　such　as　HEMP　will　cause　over-voltage　condition　inside　the　converter　valve.　The　changing　rate　of　voltage　is　so　large　that　structure　of　thyristor　may　be　destroyed.　On　the　other　hand,　when　the　thyristor　is　in　reverse　characteristic　recovery　phase,　a　large　reverse　recovery　voltage　overshoot　occurs.　This　over-voltage　condition　will　cause　damage　to　thyristor　because　thyristor　is　fragile　in　this　phase.　Based　on　these　analysis　above,　in　order　to　reduce　the　damages　caused　by　over-voltage　and　design　some　methods　to　realize　the　online-　monitoring,　this　paper　summarizes　the　fault　characteristics　of　thyristor　under　over-voltage　condition.