Based　on　the　mechanism　that　temperature　gradient　distribution　and　the　dynamic　depth-to-width　ratio　in　the　welding　pool　have　great　effect　on　the　generation　of　undercut,　a　three-dimensional　finite-element　model　is　developed　in　the　Gaussian　manner　to　simulate　the　temperature　field　of　fixed-point　TIG　welding　pool　by　the　software　ANSYS.　This　paper　simulates　the　temperature　field　of　weld　pool　on　the　condition　of　the　same　energy　input,　and　analyzes　the　analog　result.　Finally,　a　key　principle　about　the　temperature　gradient　distribution　and　the　depth-to-width　ratio　is　drawn.　From　the　simulation　results,　it　can　be　gotten　that　the　distribution　of　temperature　gradient　and　the　change　of　depth-to-width　ratio　in　the　welding　pool　have　corresponding　relationship　with　the　generation　of　undercut.