Narrow gap gas metal arc welding (GMAW) is increasingly applied in the manufacturing of thick-walled structures, such as large ships, offshore equipment, and pressure pipelines. Previous research focused on the improving weld formation and welding efficiency in this process but seldom addressed the correlations among the welding process, joint microstructure, and mechanical properties. The study aims to modify the microstructure and properties of the joint while overcoming the limitations of groove gap on cold wire swaying amplitude in deep-groove welding, thereby enhancing the practicality of the process. A rotating/swing arc narrow gap metal active gas welding assisted by a cold wire with variable swaying amplitude is proposed. Effects of arc rotation frequency, cold wire feeding speed, and the horizontal oscillation cooperative rate between the cold wire and the swing arc (η) on weld formation and welding efficiency are then investigated. Additionally, the evolution mechanisms of the microstructure and mechanical properties of cold wire-assisted rotating/swing arc narrow gap welding joints are clarified. Experimental results show that the cold wire-assisted rotating/swing arc processes yield stable weld formation and increase the welding efficiency by 25.7% and 44.2% at η ≤ 0.5, respectively. Compared to the rotating arc process, the swing arc process achieves greater penetrations into the groove sidewalls and weld bottom even at smaller swaying amplitudes of the cold wire; the swing arc has no the reheating effect on the rear of the molten pool, thereby narrowing the coarse-grain heat-affected zone (CGHAZ). This nonreheating effect, combined with the heat absorption effect of the cold wire, accelerates the molten pool cooling, substantially refining the weld grain size and toughening the CGHAZ. Owing to the dominant factors of the microstructure type and grain size, impact energy near the fusion line increases by 53.8% while weld strength rises by 6.0%. Consequently, the two cold wire-assisted processes concurrently improve welding efficiency and joint performance, advancing the application of high-quality, high-efficiency methods in narrow gap welding.