Abstract: The welding test of 26 mm thick EQ70 steel plate was carried out by using narrow gap laser wire filling welding technology.The first pass used self fusion welding as backing, and the last four passes used wire filling welding to fill the groove layer by layer.The microstructure of the joint at three positions of bottom, middle and surface layers in the thickness direction was compared and observed, and then the mechanical properties were tested.The results show that the weld is well formed on both sides with little deformation and no defects such as cracks.There are a large number of martensite structures in the weld zone and heat affected zone of self fusion welding pass.There are a large number of acicular ferrite and granular bainite structures in the weld seam of filler wire welding, and a large number of lath martensite structures in the heat affected zone.The heat affected zone of the self fusion welding and wire-filling welding near the base metal has softening phenomenon.The tensile strength of the wire filling weld is higher than that of the self fusion weld.