Abstract: Using 84 mm high-heat-input welding steel EH40 as an example,the butt welding test was given by double wires narrow gap metal active gas arc welding（ NG-MAG）. The microstructure and mechanical properties of this welding joint were tested and analyzed. The results showed that the method was suitable for high-heat-input welding steel with large thickness. There were better weld quality and no macro defects. The microstructure of HAZ consisted of lath bainite,granular bainite and acicular ferrite. The tensile strength of the joint was nearly 520 MPa,which was close to that of the base metal. The sample was bent at 180° and no obvious crack was observed on the tensile surface. The welded joints of high-heat-input welding steel EH40 with superior mechanical properties could be obtained by double wires narrow gap MAG welding process.