The Ti3Al/TC11 dual alloy bar joined by electron beam welding was deformed by near isothermal forging and then processed by gradient heat treatment. Afterwards, the mechanism of interface strengthening has been investigated according to the microstructure evolution and properties. The results show the brittle phases formed in the re-solidification can be broken by near isothermal forging; the lattice distortion energy is increased obviously due to deformation, so the nucleation ratio of recrystallization is improved, which method is fine-crystal strengthening. After the dual alloy deformed by near isothermal forging is further processed by gradient heat treatment, the room temperature tensile strength of the joint is higher than that of the Ti3Al alloy; the high temperature tensile strength of the joint is equal to or higher than that of the TC11 alloy. The reason is that the finer and phases are precipitated from the phase transus microstructure in the welding interface, which produces the effects of fine-crystal strengthening and dispersion strengthening.