The dissimilar titanium alloys Ti70/TA5 are welded by using electron beam welding. The microstructure and mechanical properties of the welded joints are systematically investigated, and the welding parameters are optimized. Results show that the fusion zone (FZ) is mainly α’ martensite, and the heat-affected zone (HAZ) in the Ti70 side consists of fine α’ martensite, residual α phase, and original β phase, while the HAZ in the TA5 side is composed of coarser α phase, serrated and acicular α phase. Transmission electron microscope (TEM) analysis demonstrates that the martensite in the FZ presents the lath-like morphology. There are high-density dislocations within martensite, which has a certain orientation relationship with the β phase. Under the appropriate welding procedure, the tensile strength of the dissimilar joint is close to that of the TA5 base metal. The joint fracture dominantly presents the characteristic of ductile fracture. During welding, electron beam scanning is beneficial to improving the solidification of molten pool and grain refinement; thus, the mechanical property of the welded joint is increased to a certain extent.