Abstract: The joints of hot-dip galvanized dual phase high strength steel(HDG-DP600) thick 2.25 mm were obtained by the resistance spot welding(RSW) process under the external constant magnetic field generated by a pair of hollow cylindrical permanent magnets,and the mechanical property,nugget size and grain structure of weld joints were analyzed.It was found that the tensile-shear force of the joints was increased by 11.01% compared with conventional RSW without the external constant magnetic field;the nugget diameter was increased by 5.20%;the crystal grain was refined and the nugget was'wide and flat'.The result indicated that the external magnetic field generated by the permanent magnets can effectively improve the quality of the joints of RSW.