Undercut　is　one　of　the　most　important　problems　which　restrict　welding　speed.　The　research　indicated　that　surface　flow　direction　of　welding　pool　near　the　periphery　is　the　most　important　reason　of　undercut　occurred.　When　the　existence　of　surface　active　element　(e.g.　the　content　of　S　is　about　0.02-0.04%　in　steel)　change　the　sign　of　surface　tension　temperature　coefficient　to　positive,　undercut　always　occurs　in　TIG　moveless　welding,　and　not　occurs　when　the　content　of　S　in　iron　is　very　low(e.　g.　S%　is　less　0.01%).　Temperature　gradient　is　the　most　important　factor　which　influences　the　depth　of　undercut.　The　depth　of　undercut　is　bigger　with　the　increase　of　temperature　gradient　when　the　other　parameters　are　the　same.　When　filling　metal　into　welding　pool,　it　restrained　the　molten　metal　movement　from　periphery　to　the　pool　center.　So　it　can　reduce　the　tendency　of　undercut,　and　eliminate　undercut　as　increasing　of　filling　metal　ultimately.