International　thermonuclear　experimental　reactor　correction　coil　cases　are　made　of　heavy,　thick,　high　strength,　and　high　toughness　austenitic　stainless　steel　316LN.　The　BTCC　(bottom　and　top　correction　coils)　case　has　the　dimension　of　2.5　x　7　m(2)　and　cross　section　of　239.8　x　146.7　mm(2),　side　correction　coil　case　has　the　dimension　of　7.2　x　7.6　m(2)　and　cross　section　of　147.8　x　168　mm(2),　and　they　will　be　closure　welded　after　winding　pack　insertion.　The　20　mm　welding　depth,　dozens　of　meter　of　welding　length,　strict　welding　requirements,　large　size,　and　complex　configuration　bring　a　big　challenge　to　this　closure　welding　work.　20　kW　high　power　laser　welding　is　selected　as　the　main　welding　method　because　of　the　advantage　of　potential　welding　deformation　control　and　its　penetrating　ability　and　cracking　resistance.　The　welding　parameter　is　developed　that　can　cover　the　assembly　gap　from　0　to　0.5　mm　with　a　good　welding　quality.　A　special　test　coupon　is　designed　for　the　welding　procedure　qualification,　and　related　tests　are　carried　out　to　qualify　the　joint　properties　of　bend,　tensile,　and　impact.　Finally,　a　full-scale　BTCC　case　is　welded.　After　welding,　ultrasonic　testing　confirms　that　almost　all　welds　satisfy　the　weld　seam　quality　requirement.　The　recorded　temperatures　less　than　250　degrees　indicate　that　the　temperature　induced　by　welding　will　not　harm　the　internal　winding　pack.　The　dimensional　deviation　of　the　inner　face　is　less　than　4　mm　and　also　satisfies　the　tolerance　requirement　of　+/-　2　mm　for　the　BTCC　case.