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Bases on a non-superplastic Al-5.40Zn-l.98Mg alloy (wt.％), we developed a new high-strain-rate superplastic alloy by applying Sc and Zr microalloying technology.The new Al-Zn-Mg-Sc-Zr alloy has a maximum superplasticity of 1520％ and can successfully achieve hemi-ellipsoidal bulge forming at 500 ℃ and 10-2 s-1.Compared with the coarse and recrystallized grains of Al-Zn-Mg alloy, the new alloy consists of ultrafine sub-grains and strong β-fiber rolling texture.During deformation, the grains of Al-Zn-Mg alloy further grow up, whereas for the Sc-enriched alloy, the low angle grain boundaries gradually transfer into high angle grain boundaries and the grains gradually orientate randomly.The results reveal that dislocation viscous glide/climb creep is dominant during the hot deformation of Al-Zn-Mg alloy.