The narrow-gap solid fiber laser welding with filling hot wire replaces the previous narrow gap tungsten inert gas (TIG) welding. The thickness of the welding is 20 mm and the material is SUS304 austenitic stainless steel. Preliminary study shows that the groove form and the welding process parameters are suitable for narrow gap laser welding with filling hot wire. The causes of blowhole and crystallization crack in the weld metal are analyzed through experiments and the process parameters are adjusted. Under the optimum process parameters, smaller heat affected zone, better surface formability are obtained, and there is no welding defects in the welded joints. The weld metal is fine austenite columnar crystals, which forms a good connection with the base material. Welding deformation is small enough to satisfy the requirement. The weld metal is slightly micro harder than the base metal. The tensile and bending properties meet the test requirements, obtaining good mechanical properties of the welded joints.