Abstract: Mechanical properties and microstructures of S355 steel welded joints which were gotten from Ar+CO2 gas shielded arc welding were studied via tensile,impact and metallographic test etc..The results showed that the joints of S355 steel welded with wire JM-56 had fine shear strength and mechanical properties.Microstructure in the welded seam mainly contains pro-eutectoid ferrite,acicular ferrite and a little pearlite as well as granular bainite．The fusion area and overheated zong of heat-affected zone mainly contains pro-eutectoid ferrite,some pearlite and definite quantity granular bainite.