Abstract: In order to exploring the method to control the welding quality of thin-wall welding tube of tantalum during the production, the experimentation of TIG helium-arc small gap spot-butt welding of tantalum sheet and corresponding numerical simulation were studied. The dynamic size of molten was successfully measured with the high-speed vidicon of Fastcam 10Kc. The numerical simulation adopted ANSYS and three-dimension conducting-heat model in welding was set up. The effect of butting gap and clamp were synthetically considered, the result of calculation and that of the experimentation were identical better. The influence of the welding parameters on molten pool were studied by numerical simulation. The results showed the gap enhance the molten pool being ellipse. The appearing time and grow-up speed of molten pool and critical current from the calculation had the important instructional significance for optimizing the welding parameter.