The　reduced　activation　ferritic/martensitic　CLF-1　steel　used　in　the　nuclear　fusion　reactor　test　blanket　module　(TBM)　is　welded　by　15　kW　fiber　laser,　and　the　penetration　welding　of　17.5　mm　thick　CLF-1　steel　is　realized.　A　sound　joint　forming　can　be　obtained　by　optimizing　the　process　parameters,　and　there　are　no　obvious　defects　such　as　cracks　and　pores　within　the　welded　joint.　Furthermore,　the　microstructure　and　mechanical　properties　of　the　welded　joints　are　analyzed.　The　experiment　results　show　that　the　microstructure　of　the　weld　mainly　contains　the　thick　lath　martensite.　The　microstructure　of　the　heat　affected　zone　mainly　contains　the　tempered　sorbate　and　martensite　dual　phase　structure　caused　by　secondary　tempering　of　welding　thermal.　At　the　average　room　temperature　and　550　,　tensile　strength　of　welded　joint　is　very　high.　The　fracture　position　is　located　at　the　base　metal.　The　microhardness　of　the　weld　is　significantly　higher　than　that　of　base　metal,　and　there　is　no　obvious　soft　zone　at　the　heat　affected　zone.　The　weld　joints　exhibit　favorable　impact　toughness,　so　it　has　very　good　comprehensive　mechanical　properties.　©　2018,　Editorial　Board　of　Transactions　of　the　China　Welding　Institution,　Magazine　Agency　Welding.　All　right　reserved.