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°.