Abstract: The mid-section Ω weld of the Control Rod Drive Mechanism (CRDM) in the "HPR1000" nuclear power unit serves as a critical sealing structure, requiring zero leakage under high-temperature, high-pressure, and intense irradiation conditions throughout the reactor’s 60-year service life. Its on-site welding faces significant challenges including confined space and stringent quality requirements. To address these issues, a high-precision integrated automatic welding system was developed. This system achieves positioning, wire-feeding, and current control accuracy better than ±1%, with arc voltage control precision of ±0.1V. Employing a modular design, it enables seamless switching between self-fusing and wire-filling welding processes by replacing the wire-feeding module and front gas shield. Dedicated self-fusing and wire-filling welding processes were synchronously developed for CRDM mid-section Ω welds. Extensive simulated welding and repeated cutting-repair welding tests validated the stability of both processes. Results demonstrate that welds produced by both techniques exhibit smooth, well-contoured bead surfaces, appropriate backside reinforcement, and excellent fusion with the base metal. Metallographic examination revealed typical dendritic microstructures, with all joints meeting relevant quality standards. This study provides key technological support for the localization of CRDM welding technology and batch construction of "HPR1000" units.