The low-alloy and high-strength steel D36 is joined by narrow-gap laser wire-filled welding, which results in good multi-layer and multi-pass thick plate welding without cracks, pores, and other defects after welding. The microstructure and mechanical properties of the welded joints are analyzed by the tension and bending test, microhardness test, microstructure observation, and other analytical methods. The results show that the coarse-grained heat-affected zone consists of a large number of lamellar martensite, and the microstructure of the fine-grained zone is composed of the fine-grained pearlite and ferrite. The weld center structure is proeutectoid ferrite + acicular ferrite + granular bainite. The regions with filler wire weld bead hardness in order from high to low is heat affected zone, weld center, and base material. The regions with laser primer welding hardness in order from high to low is weld center, heat affected zone, and base metal. The tensile specimens of the welded joints all fractured at the base metal position. The fracture is at an angle of 45° with the force direction, and there is obvious necking, which is a typical ductile fracture. The average tensile strength of the welds is 537 MPa, and the average elongation after fracture is 15.6%.