5B70 aluminum alloy plates with a thickness of 1.5 mm were welded using tungsten inert gas (TIG) welding with 5B71 filler wire. The mechanical properties and microstructures of the welded joints were investigated. The results show that under appropriate welding parameters, joints with excellent mechanical properties could be achieved. The tensile strength reached 342–350 MPa, the joint strength coefficient was 0.82–0.84, and the elongation at fracture was 9.5%–13%. The weld hardness ranged from 87HV to 93HV, with the minimum hardness located near the fusion line. The hardness profile exhibited a double V-shape. The fracture surface displayed a 45° shear morphology, characterized by a mixed mode of dimples and cleavage facets, with strengthening phase particles observed within some dimples. The fusion line of the joint was distinct, and the microstructures differed across the weld zone, fusion line, heat-affected zone, and base metal. The weld zone consisted of refined equiaxed grains with nonuniform sizes.