Abstract: This study systematically investigates the mechanical properties and fatigue characteristics of AISI 4130 chassis steel tubes under TIG welding processes, focusing on the U20 roll cage welding right-angle butt joint specimens from the China SAE Baja Competition. A thermomechanical coupled finite element model based on the Goldak double ellipsoid heat source was constructed to dynamically simulate the welding temperature field and residual stress field using ANSYS Workbench. A multiaxial fatigue life evaluation system integrating the mean stress-corrected Morrow model and the E-N curve method was established via nCode Design Life. Innovatively adopting response surface methodology to optimize welding parameters, a three-factor three-level orthogonal experiment revealed that optimal weld quality is achieved at a torch speed of 3 mm/s, welding duration of 20 s, and power density of 40 W/mm². Under these conditions, the weld penetration reached 3.2 mm, with an 18% improvement in weld width uniformity, a residual stress peak reduction to 285 MPa, and a maximum deformation of 0.08 mm, representing a 23% enhancement in forming quality compared to conventional parameters. Microstructural analysis indicated refined grain sizes (12–15 μm) in the weld zone and a 37% reduction in hardness gradient within the heat-affected zone. Multibody dynamics-based load spectrum analysis demonstrated spatially graded fatigue life distributions: the minimum life of 7.37×10^7 cycles occurred at 15 mm from the loading point, while regions beyond 30 mm exhibited a maximum life of 7.311×10^8 cycles, compliant with SAE J1099 standards. Experimental validation confirmed a prediction error of less than 9.2% for the finite element model. The study establishes a quantitative mapping relationship among welding parameters, microstructural properties, and fatigue life, providing a damage tolerance design-based framework for selecting process windows in high-strength steel racing chassis optimization.