Reduced　activation　ferritic　martensitic　(RAFM)　steel　is　chosen　as　a　structural　material　for　test　blanket　modules　(TBMs)　to　be　constructed　in　International　Thermonuclear　Experimental　Reactor　(ITER)　and　China　Fusion　Engineering　Test　Reactor　(CFETR).　Chinese　specific　RAFM　steel　named　with　CLF-1　has　been　developed　for　CFETR.　In　this　paper,　a　narrow-gap　groove　laser　multi-pass　welding　of　CLF-1　steel　with　thickness　of　35　mm　is　conduced　by　YLS-15000　fiber　laser.　Further,　the　microstructures　of　different　regions　in　the　weld　joint　were　characterized,　and　tensile　impact　and　micro-hardness　tests　were　carried　out　for　evaluating　the　mecharical　properties.　The　results　show　that　the　butt　weld　joint　of　CLF-1　steel　with　a　thickness　of　35　mm　was　well-formed　using　the　optimal　narrow-gap　laser　filler　wire　welding　and　no　obvious　defects　was　found　such　as　incomplete　fusion　cracks　and　pores.　The　microstructures　of　backing　layer　is　dominated　by　lath　martensites　and　the　Heat-Affected　Zone　(HAZ)　was　mainly　filled　with　two-phase　hybrid　structures　of　secondary-tempering　sorbites　and　martensites.　The　filler　layer　is　similar　to　the　backing　layer　in　microstructures.　In　tensile　tests,　the　tensile　samples　from　different　parts　of　the　joint　all　fractured　at　base　metal　(BM).　The　micro-hardness　of　weld　metal　(WM)　was　found　to　be　higher　than　that　of　BM　and　the　Heat-Affected　Zone　(HAZ)　exhibited　no　obvious　softening.　After　post　weld　heat　treatment　(PWHT),　it　can　be　observed　that　the　fusion　zone　of　the　autogenous　welding　bead　and　the　upper　filling　beads　mainly　consist　of　lath　martensites　which　caused　the　lower　impact　absorbing　energy.　The　HAZ　mainly　included　two-phase　hybrid　structures　of　secondary-tempering　sorbites　and　martensites　and　exhibited　favorable　impact　toughness.　(C)　2018　Elsevier　B.V.　All　rights　reserved.