The welding technology of ultra-narrow gap wire filler welding could improve the general properties of welded joints when the filler metals could change the microstructure and chemical elements. This paper focused on the 20 steel-Q345C pipe welded joints properties which welded with the ultra-narrow gap wire filler welding and the tensile strength,bending properties,impact properties and microstructure were studied. The results showed that the tensile strength,bending properties and impact toughness were all in accordance with the standard. Because the higher heat input and faster cooling speed with quench trend then the weld showed the highest microhardness. The microstructure of HAZ was composed of Widmanstatten structure,while the main components of the weld center were proeutectoid ferrite precipitated along grain boundary and acicular ferrite and pearlite in grain boundary. The microstructure of the base metal was lamellar pearlite and ferrite. So the ultra-narrow gap laser wire-filling welding technology for 20 steel-Q345C pipe has good engineering application value.