Based on the thermal elastic-plastic FEM （Finite Element Method）, the thermody-namics model of laser welding has been set up, by three dimensional ANSYS finite software element, with APDL language Supported by ANSYS, FES （Finite Element Simulation） of temperature field, stress field in laser welding of BT20 Titanium Alloy Mouthpiece is studied. Combined with numerical calculation results, after in-stantaneous air-cooled handing during laser welding, the stress distributed situation of specimen has been analyzed.