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.