Abstract: S355 hot-rolled H-beam was welded by SAW. The microstructure of the welded joint was observed by optical microscope and scanning electron microscope. The microhardness and low temperature impact property of the welded joint were tested according to requirements of European standards, and the fracture morphology was observed by scanning electron microscope. The results show that the microstructure of weld zone is mainly composed of acicular ferrite and pearlite, the microstructure of heat-affected zone（HAZ） is composed of coarsegrained zone, fine-grained zone and incomplete recrystallization zone, the coarse-grained zone is mainly composed of coarse ferrite and pearlite, and a small amount of massive ferrite and granular bainite, the fine-grained zone is composed of fine ferrite and perlite. The microstructure of partially recrystallized zone is composed of fine ferrite,pearlite and banded structure of original base metal. The Vickers hardness of the welded joints is less than 185HV10which uniform distributed in all areas of welded joints, and the impact toughness at-40℃ of the welded joints meets the requirements and it is highest at the fusion line position and decreases at all other positions.