Application　of　aluminum　alloy　heavy　section　is　limited　for　its　worse　weldability,　low　welding　efficiency　and　large　welding　deformation.　5083　aluminum　alloy　plates　of　25　mm　thick　are　welded　using　IPG　YLS-6000　fiber　laser　and　ultra-narrow　gap　fiber　laser　beam　welding　method.　The　microstructure　and　mechanical　properties　of　joint　are　tested　by　optical　microscope,　scanning　electron　microscope　and　tensile　testing　machine　for　low　temperature.　The　results　shows　that　defect　free　butt　joints　with　7　layers　are　obtained　with　ultra-narrow　gap　laser　welding.　The　width　of　weld　is　uniform　and　less　than　4.5　mm.　Microstructure　of　weld　consists　of　columnar　grain　and　some　discontinuous　dot-like　precipitated　phases.　There　are　no　obvious　coarse　grains　but　some　precipitation　in　heat　affected　zone　(HAZ).　The　microhardness　of　the　HAZ　is　slightly　lower　than　that　of　weld　metal　and　base　metal,　and　the　microhardness　of　base　metal　is　the　same　to　that　of　the　joint.　The　tensile　strength　of　joint　increases　a　bit　when　temperature　decreases　(such　as　at　273　K,　243　K,　213　K　and　183　K),　which　performs　as　well　as　the　base　metal.　Samples　are　bended　by　180°　without　large　defects　observed,　which　shows　the　joint　has　excellent　plasticity.