Abstract: The stress-strain curves of the hot rolled 5A90 Al-Li alloy fine-grained sheets at the deformation temperature of 375-475 ℃ were tested by the tensile test with strain rate of 0.0004-0.05 s^-1, and the Arrhenius constitutive model was established. The grains and distribution of grain boundaries were observed at different deformation temperatures, strains and strain rates by electron back sacttering diffraction（EBSD）, and the microstructure evolution of 5A90 fine-grained sheets during high temperature deformation was analyzed. The results show that the average grain size of the original 5A90 hot rolled sheets is only 1.27 μm, and the elongation at 375℃ and 0.002 s^-1is as high as 607.9%, showing excellent low temperature superplastic deformation ability. When the deformation temperature is 375-425℃, the activation energy is113.72 kJ/mol,the superplastic deformation mechanism is grain boundary slip and dislocation slip; when the deformation temperature is higher than 450℃, due to the temperature effect, the grains grow abnormally, the superplasticity deformation ability of the fine-grained sheets decreases, and the deformation resistance increases. The suitable superplastic deformation condition of hot rolled 5A90 fine-grained sheets is the deformation temperature of 375-425 ℃ and the strain rate of0.002-0.01 s^-1.