Abstract: Objective When performing I-groove narrow gap groove welding, the use of backings was usually involved. Different materials of backings would have an influence on microstructure and mechanical properties of root weld by narrow gap welding. In order to study effects of different backings on welding process, in this study, narrow gap oscillating arc GMAW combined with ceramic, copper, and steel backings was to used weld 50 mm thick ZG30SiMn/NM450 (short for ZG/NM) combination in middle trough. Methods After welding, root weld was observed for microstructure and tested for hardness, tensile strength and other mechanical properties. Results The experimental results showed that under the action of oscillating arc, incomplete fusion defects on the sidewalls of the weld were successfully suppressed, demonstrating good adaptability of narrow gap oscillating arc GMAW to ZG/NM combination. Due to different materials of backings, there were differences in width and microstructure of heat-affected zone in root weld, as well as in microstructure of weld. Width of heat-affected zone became narrow with the increase of thermal conductivity of backings, grain size gradually decreased, and content of blocky ferrite in the weld gradually decreased. The lowest hardness was found in the center of weld, ceramic backing pad had the lowest hardness in the center of weld due to differences in cooling rate, while water-cooled copper had the highest hardness. High strength of water-cooled copper root weld was attributed to its lower content of blocky ferrite. Conclusion Through the application of narrow gap oscillating arc GMAW technology and different backing materials, this study provided valuable experimental results for the welding of ZG/NM combination, and offered a thorough analysis of performance and microstructure of welded joints, providing a reference for further research in related fields.