In　this　paper,　electron　beam　butt　welding　of　the　reduced-activation　ferrite/martenstic　CLF-1　steel　with　a　thickness　of　32　mm　was　performed.　Furthermore,　the　impact　toughness,　microstructure,　phase　compositions　and　impact　fractures　of　the　joints　with　different　preheating,　post　welding　heat　treatment　(PWHT)　and　heat　inputs　were　investigated　systematically　and　the　influencing　rules　and　mechanisms　were　analyzed.　The　experimental　results　shown　that,　with　the　optimization　of　the　processes　parameters,　well-formed　joints　without　any　defects　such　as　pores,　incomplete　fusion,　and　cracks　was　obtained　by　electron　beam　welding,　which　also　possessed　favorable　tensile　and　flexural　performances.　The　impact　absorbed　energy　of　the　joints　was　not　remarkably　improved　with　preheating　or　post　welding　heat　treatment.　As　the　heat　input　dropped　from　2805　J　mm(-1)　to　1440　J　mm(-1),　the　impact　absorbed　energy　of　the　weld　increased　from　3.3-12.5　J　to　275　J　and　delta-ferrite　content　reduced　from　2.79%　to　0.75%　after　PWHT　at　740　degrees　C　for　4　hours.　At　a　heat　input　of　1836　J　mm(-1),　the　impact　absorbed　energy　of　the　weld　was　comparable　to　the　value　of　BM,　and　the　specimens　were　not　fractured　after　impact　test.　Preheating　and　PWHT　only　changed　the　size　of　martensite　laths　and　carbides　without　eliminating　the　residual　delta-ferrites　in　the　weld.　In　addition,　the　impact　fractures　exhibited　brittle　fracture　characteristics.　As　the　heat　input　reduced,　the　content　of　delta-ferrites　in　the　weld　decreased　significantly,　the　martensite　laths　in　the　weld　zone　after　thermal　treatment　were　refined,　and　Ta-rich　MX　carbides　in　the　laths　were　precipitated.　Therefore,　the　impact　toughness　of　the　joints　increased　significantly　and　the　impact　fractures　exhibited　brittle　fracture　characteristics.　(C)　2020　Elsevier　B.V.　All　rights　reserved.