<p>The narrow gap gas tungsten arc welding (NG-GTAW) horizontal welding method serves as an ideal process for circumferential seam butt welding of thick-walled cylindrical components. During horizontal welding, gravity causes the molten pool to flow downward, resulting in an asymmetrical weld formation. This can lead to defects in multi-pass welding, such as incomplete penetration and lack of interlayer fusion. A novel rotating arc narrow gap GTAW horizontal welding method has been developed that utilizes a rotating arc as the heat source to alternately heat the upper and lower sidewalls, thereby achieving reliable sidewall penetration. The rotating arc generates asymmetric arc forces that act on the molten pool at different positions during periodic rotations, effectively optimizing the fluid flow patterns. A molten pool image acquisition system integrating a high-speed camera and laser backlighting was established, employing tracer particles to characterize molten pool flow behavior. The mechanical stirring effect induced by the dynamic variation of the rotating arc force effectively enhances the flow of the molten pool, preventing excessive accumulation along the lower sidewalls and achieving uniform weld formation. Welding current and welding speed determine heat input, with sufficient arc energy being crucial for sidewall fusion in narrow gap welding. Increasing the rotation speed of the tungsten electrode enhances the arc stirring effect, facilitating more homogeneous weld formation.</p>