Taking S355 J2 N steel as the research object, the ultra-narrow gap laser welding is used for welding technology test to study the tensile strength, bending property, impact strength, microhardness and microstructure of welded joints. The results show that the all these properties of welded joints meet standard requirements. The microstructure of the base metal is composed of ferrite and pearlite. The microstructure of the weld zone is composed of first eutectoid ferrite which distributes along columnar crystals, the carbon-free bainite grows into the crystal parallel to the boundary and the acicular ferrite,granular bainite and pearlite distribute in the crystal. The reticular crystal boundary of the HAZ is obvious and its microstructure is lath martensite and M-A microstructure.The microstructure of the interlayer of multilayer are observed to find that white ferrite is precipitated along the grain boundary of primary austenite and the acicular ferrite, granular bainite and pearlite distribute in the crystal. So the ultra-narrow gap wire filler welding has good application value for the S355 J2 N steel welding.