Abstract: High temperature tensile test of 5A90 aluminum lithium alloy electron beam welding joints was carried out, using optical microscope to observe the microstructure evolution of the sample during deformation, and the deformation mechanism was analyzed. The results showed that the superplastic deformation mechanism at the initial stage of superplastic deformation mainly depended on the grain boundary migration caused by diffusion, and the fine equiaxed grains of the weld seam grew rapidly. When strain was greater than 100%, large grains in the joint began to arise dynamic recrystallization, and superplastic deformation mechanism changed into dynamic recrystallization mechanism. During superplastic deformation, the average grain size of the heat affected zone was close to the average grain size of the weld, and there was a homogenization process in the microstructure. Homogenization coefficient K was used to characterize the degree of homogenization of the weld and heat affected zone. With the deformation, the K value increased gradually.