A heterojunction of 7075 aluminum alloy and TC4 titanium alloy was obtained by vacuum electron beam technique. The macroscopic morphology, microstructure and element distribution of the welded joints were observed by metallographic microscope, scanning electron microscope and energy spectrum analyzer. The formation process of intermetallic compounds in the transition layer and its influencing factors were also analyzed. The test results show that the microstructure of the aluminum alloy side of the fusion zone is dominated by columnar crystals and equiaxed grains, and a small amount of fine–grained structure is formed. The α–phase and α' phase of the needle–like or lamellar phase are precipitated on the titanium alloy side. Since the free energy of TiAl₃ is the smallest, the main component of the intermetallic compound formed at the interface between the aluminum weld and the titanium weld is TiAl₃. The insufficient diffusion of elements leads to the formation of a small amount of intermediate phases such as TiAl and TiAl₂, and increases with the heat input. The intermetallic compound layer gradually thickens, but the type of the compound does not change. The test results provide reference for the study of welding interface of aluminum–titanium dissimilar alloy.