Abstract: The superplastic deformation behavior of electron beam welded joints of 5A90 aluminum lithium alloys was researched by high temperature tensile test. The results indicated that the electron beam welded joints of 5A90 aluminum lithium alloys present good superplasticity. The peak flow stress decreased while the temperature increased or the initial strain rate decreased, and the flow stress was below 32 MPa while the initial strain rate was below 1×10²/s. The elongation of joints increased first and then decreased with increase of the deformation temperature and initial strain rate, and the maximal elongation of 171.1% was obtained at the optimal parameter 450℃, 5×10³/s. Deformation coefficient K was introduced to characterize the superplasticity of electron beam weld joints. The K values were beyond 70% at all deformation parameters.