Abstract: Two welded joints of EH40 micro-alloyed steel were produced by narrow gap GMA welding with two welding parameters. The effect of filling thickness on the distribution of coarse-grained subregions in HAZ was analyzed and the mechanical properties of welded joints were studied. The research result showed that swing arc narrow gap MAG was suitable for narrow gap welding of microalloyed steel and good formation of weld was obtained without the defect,such as lack of fusion. Distribution of coarse-grained subregions was a periodic arrangement of UACG-HAZ,SCRCG-HAZ,IRCG-HAZ and CG-HAZ with a large filling thickness,while distribution of coarse-grained subregions was mainly a periodic arrangement of SCRCGHAZ and IRCGHAZ with a small filling thickness. All tensile specimens were broken at the base material,and the impact energy at room temperature could meet the requirements. The swing arc narrow gap MAG process could obtain MAG welded joint of EH40 steel with good mechanical properties.