The microstructural evolution and mechanical properties of a vacuum electron beam welded aerospace 5B70 aluminum alloy joint were studied. Quantitative analyses of the phase composition, microstructural evolution, grain size, grain boundary density, and texture changes were performed by X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction. The fusion zone (FZ) comprises equiaxed cellular crystals, and a fine ~20 μm-thick crystal layer forms in the transition zone (TZ) between the FZ and heat affected zone (HAZ). The HAZ closely resembles the base material (BM), retaining the original rolling microstructure. Mechanical property testing shows that the fine-grained layer in the TZ exhibits the highest nanohardness, with the FZ corresponding to the lowest microhardness. The welded-joint sample has lower yield strength, ultimate tensile strength, and elongation after fracture than the BM. These reductions of mechanical properties are primarily influenced by the grain size and distribution of the precipitated phases.