<p>The configuration of the tungsten cathode significantly impacts the tungsten inert gas welding (TIG) arc characteristics and molten pool behavior. This study developed an isosceles trapezium plate-shaped tungsten cathode with a rectangular cross-section for narrow gap welding. A 3D dynamic model was created, including the tungsten cathode, arc column, and substrate metal. By solving the physical fields of arc-molten pool coupling system, the variation laws of the arc-molten pool coupled physical fields were obtained with different tip shapes of cathode. The results indicate that the isosceles trapezium plate-shaped tungsten cathode tip configuration plays a major role in regulating the arc discharge gap. The platform length of isosceles trapezium plate-shaped tungsten cathode in the head-on orientation is longer, which enables a larger current density distribution range. Consequently, the distribution range of Lorentz force, cathode jet and arc temperature also expand in this orientation. This leads to an expansion of the thermal distribution range on the molten pool surface, and the weld width is increased. As the tip angle increases, the discharge gap decreases in the flank orientation. This expands the cathode tip current density range but reduces current density. Consequently, the arc temperature, cathode jet and Lorentz force decrease, yet their distribution range increases. This change can weaken the thermal effects on the surface of the molten pool, and molten pool became shallower. However, the thermal distribution range on the molten pool surface further is expanded, and molten pool width is increased.</p>