Abstract: According to the basic principles of fluid mechanics, tracking the free surface by using VOF method, a three dimensional mathematical model for fixed-point A-TIG welding was established. Considering the buoyancy, electromagnetic force, arc pressure and surface tension of liquid metal in the molten pool, as well as the convection, radiation and heat transfer, the FLOW-3D software was used to simulate the fixed-point A-TIG welding molten pools' free surfaces, temperature fields and flow fields, in particularly, the free surfaces of the molten pools with or without active flux was analyzed. The results show that in case of the process without active flux addition, the free surfaces' centers are concave surrounded by convex surface, while in case of the process with active flux, surface tension temperature coefficient changed from negative to positive, thus free surfaces' centers are convex, and the periphery is concave. When the welding current is 150 A or less, the simulated result of the molten pools' temperature fields, flow fields and shapes with the consideration of free surface are consistent with the experimental results, as well as the simulated process without considering free surface.