The invention relates to a thick-plate narrow-gap laser scanning filler wire welding method. The method comprises the steps: firstly arranging an I-type beveled edge or a deep U-type beveled edge with a thick truncated or a deep V-type beveled edge between two thick plates to be welded, transmitting laser beams to a weld seam by adopting a scanning galvanometer, carrying out the bottoming by virtue of laser self-melting welding, and then carrying out the laser scanning filler wire welding. In the filler wire welding process, the width of the beveled edge is detected in real time by adopting a visual detection system, and the deflection angle of the scanning galvanometer is further controlled, so that the laser beams can scan back and forth at two sides of the weld seam, a weld wire stretches into a gap of the beveled edge from the front of the laser beams, and the weld wire is molten by the laser which scans back and forth; the scanning galvanometer is driven by adopting a plane travel cart or an annular guide rail to move along the weld seam, and the scanning galvanometer is adjusted as the increment of welding beads, so that the laser can be focused in the plane of the weld seam to melt the weld wire. By adopting the thick-plate narrow-gap laser scanning filler wire welding method, the welding heat input can be reduced, the weld deformation can be alleviated, a thick-plate weld joint with small weld deformation and good gap side wall fusion can be obtained, and the welding efficiency and welding quality of the thick plate can be greatly improved.