With the consideration of the influence of arc driving forces, a 3D transient numerical model was established for the ripple formation process during pulsed tungsten inert gas (TIG) welding. To avoid the influence of droplet caused by the melting of welding wire on the surface fluctuation of molten pool, the wire-feeding process did not proceed when the arc was scanning the 2024 aluminum alloy substrate. Results show that the molten pool surface fluctuates periodically when the current is switched between the base value and the peak value. With the arc movement, the rear side of molten pool is solidified gradually. The molten pool surface is solidified before flattening under the cooling with large temperature gradient, resulting in the welding ripples. The formation frequency of the welding ripples is equal to the current pulse frequency. The distance between adjacent welding ripples is approximately equal to the product of the arc scanning velocity and the current pulse frequency.