The effects of welding parameters such as spot diameter, gap width, laser power and welding speed on the joint formation of ultra-narrow gap laser beam welding with filler wire are studied by using IPG YLS-6000 fiber laser and defocused beam. Results show that when the spot diameter is wider than gap width and leading filler wire and filler wire melting in molten pool are used, the joint formation in heat conduction welding model is the best; smaller spot diameter leads to and larger spot diameter leads to pores, excessive fusion of sidewalls. Under the same welding conditions, the greater the gap width, the narrower range of welding process windows for welded joints without defects will be. As the gap width increases, the suitable laser power is increased, and the welding speed is reduced. Small heat input can produce lack of fusion on sidewall, and larger heat input can cause keyhole induced pores. Under the optimized parameters, 60 mm thick stainless steel plates are welded by less than 5 mm ultra-narrow-gap laser welding with filler wire, and a complete defects free butt joint which consists of 20 layers is obtained. The width of weld is uniform and only about 3.8 mm, and the angular distortion of the joint is about 0.6°.