<p>A new computational fluid dynamics model of the submerged arc welding pool has been developed, incorporating the coupling of slag viscosity. The flow field and temperature field of the weld pools were compared under two conditions: with air and slag as the viscosity values of the medium domain, respectively. It is demonstrated that the slag viscosity effectively restrains the spreading flow of molten metal at the surface, resulting in a maximum width of the weld pool that is only 64 pct of that observed under air viscosity. In this case, high-temperature molten metal accumulates more in the center of the weld pool, thereby intensifying downward flow and augmenting weld penetration by 59 pct in comparison to the conditions with air viscosity. This intensified internal flow ensures uniform temperature distribution within the weld pool and decelerates the solidification process of molten metal at its end. Comparison between simulation and experimental results confirms that the model provides a more accurate simulation of the submerged arc welding bead profile. This study elucidates the exact mechanism of how slag viscosity influences the submerged arc welding pool and may also offer fundamental insights for optimizing the design of welding flux.</p>