Abstract: As the demand for durability of steel structure bridges continues to rise, the requirements for the fatigue resistance design of steel bridge decks have become increasingly stringent. Addressing the penetration welding technical requirements for the hot-rolled variable-thickness U-shaped ribs in the steel bridge deck of the Xiamen–Kinmen Bridge (Xiamen section), this paper conducts research from two perspectives: the development of new welding processes and the development of new welding equipment. On one hand, through welding process trials, an efficient full-penetration welding process for hot-rolled variable-thickness U-shaped ribs based on double-sided bevels has been proposed; for the internal weld seams, either solid wire submerged arc welding or gas shielded arc welding is employed, while the external welding utilizes submerged arc welding; on the other hand, a novel flexible internal welding machine for U-shaped ribs has been developed, and the seam tracking system for specialized external welding machines for multi-head gantry-type U-shaped ribs has been improved. The research results have significantly enhanced the welding quality and efficiency of the hot-rolled variable-thickness U-shaped ribs in steel bridge decks, while substantially reducing welding deformation. Furthermore, from the perspective of welding quality control, this study proposes recommendations for optimizing the acceptance criteria for weld seam quality.