Abstract: When top fusion zone radius and plate thickness are constant, the three-dimensional shape model at different back fusion zone radius was obtained by analyzing three-dimensional liquid surface of TIG (tungsten inert-gas)welding full-penetrated molten pool with the arc.The effect of the arc on liquid surface of TIG welding full-penetrated molten pool was investigated through numerical simulation with finite element analysis software (Surface Evolver). The results show that the size and shape of the back liquid surface of molten pool aren' t influenced remarkably by the arc. However, when the back fusion zone radius is small (about less than or equal to 2.5 mm), the effect of the arc on the characteristic parameters of the top liquid surface is obvious, and the top liquid surface is depressed by the arc.From the pulse base value to the pulse peak value, the arc voltage or the arc light intensity is increasing, which is due to not only the current increasing but also top liquid surface of the molten pool being depressed.