Abstract: To overcome the challenges in welding small-diameter dissimilar metal tube joints made of GH4202 superalloy and 1Cr18Ni9Ti stainless steel—such as difficulties in controlling weld bead formation, significant heat accumulation, and high costs associated with process optimization—this study established a thermal-mechanical coupled numerical model based on the finite element method. By simulating and analyzing the temperature field during welding, the influence of tungsten electrode offset on weld formation was investigated, and the heat accumulation effect within the temperature field was examined. The results show that the simulated molten pool morphology is in good agreement with experimental observations. Shifting the tungsten electrode towards the superalloy side significantly increases the penetration width on both the outer and inner walls. However, to ensure proper fusion at the root face, the offset must be controlled within 0.4 mm. To ensure stable and optimized weld formation, a segmented current process is required to achieve a uniform weld bead. The findings provide a theoretical basis and data support for the welding process design of dissimilar metal tube structures in aerospace applications.