Abstract: The experiments of TIG butt welding process for 1 mm tantalum thin plate under different gap is carried out with shielding gas of helium,and the dynamic behavior of the molten pool was observed by high-speed camera. The effects of process parameters on temperature field and displacement field were studied by means of numerical analysis,and three kinds of forming mechanism of molten pool of TIG butt welding of tantalum sheet at different gaps were obtained. At the gap of δ≤0. 108 mm（ δ is butt gap）,the forming mechanism of molten pool is the same with that without gap. When the gap is 0. 108 ～ 0. 22 mm,the droplets formed on the two butt workpiece and the workpiece continue to expand to the butt gap,and the molten pool formed when the two workpiece contact; When the gap is 0. 22 ～ 0. 35 mm,there is still a large gap when the droplets formed,molten metal contracted and accreted on the edge of workpiece because of surface tension. With the increase of heat energy,the gap was decrescent and the volume of molten metal was increscent. When surface tension in direction of slope is lower than gravity,the molten metal flowed down and formed the molten pool between the gaps.