Abstract: Abstract: Laser narrow gap wire filling welding was carried out on 45mm thick TC4 titanium alloy. The microstructure of the welded joint was finely characterized using OM, SEM, and TEM methods, and the microhardness was tested and analyzed. The results indicate that 45mm thick TC4 ultra narrow gap welding can achieve defect free welding joints. The base self melting weld seam consists of coarse equiaxed grains with needle shaped α arranged in a mesh basket; the coarse grain zone in the heat affected zone consists of columnar crystals of intergrown crystals, with finer needle shaped α within the grains; The grain size in the fine grain zone of the heat affected zone is slightly larger than that of the base material, and the alpha size is equivalent to that of the base material. Fill the weld bead, which is a layer of interwoven alpha, with fine equiaxed grains in the coarse grain zone of the heat affected zone, and significant growth of alpha layers within the grains; The alpha size of the fine grain zone in the heat affected zone is significantly larger than that of the base material, and the phase boundary is blurred. The microhardness of the welded joint has the highest heat affected zone, which is 340-350HV0.2; The lowest hardness of the base metal is 280-300 HV0.2, with the microhardness of the self melting weld being higher than that of the filling weld.