Electron beam welding experiments of TiAl and TC4 titanium alloy under different welding parameters were carried out. The influence of aluminum content in the weld on the microstructure and mechanical properties of the joints were analyzed. The weld mainly consisted of the α2-Ti3Al phase and α-Ti phase, with a small quantity of B2 phase and YAlx phase when the beam acted on the contact face. In the joints the content of molten TiAl and TC4 was about 1/3 and 2/3, and the percentage of Al was about 28 wt%. This high content of Al led to the formation of a brittle α2 phase, thus the ductility and toughness of the joints decreasing. In the centered beam welding, the tensile strength of the joints was low. The fracture of the joints was the classic brittle transgranular and quasi-cleavage fracture. When the beam was deflected towards the TC4 alloy, the Al content in the weld decreased and the mechanical properties of the joints improved. When the lateral deviation hs was 0.2?mm, the highest tensile strength of the joints reached 422.2?MPa.