Many　structures　in　the　vacuum　chamber　in　the　International　Thermonuclear　Experimental　Reactor　(ITER)　were　welded　thick　stainless　steel.　For　thick　plates,　traditional　welding　technologies　often　cause　large　distortion,　pores,　or　incomplete　fusion　on　the　side　walls.　This　paper　thus　explores　ultra-narrow-groove　laser　welding　in　conduction　mode　with　welding　wires　using　a　fiber　laser.　Results　show,　while　the　ultra-narrow　groove　significantly　reduces　the　distortion,　the　concurrent　heating　from　the　laser　beam　on　the　side　walls　and　bottom　of　the　groove　ensures　side　wall　fusion.　The　conduction　mode　eliminates　porosity.　Defect-free　butt-joint　welds　were　made　on　60-mm-Thick　stainless　steel　plates　with　a　3-mm-wide　ultra-narrow　groove　in　20　layers.　The　distortion　angle　was　only　0.6　deg.　Stainless　steel　with　a　thickness　and　groove　width　of　120　and　4.5　mm　can　be　welded　using　the　method　and　experimental　conditions.