Superplasticity and deformation mechanism of Mg-7.0Al-0.2Zn（AZ70）alloys without typical fine-grained structure was investigated. It is indicated that the AZ70 alloy exhibits superior superplasticity and the maximum elongation of 191.5% was obtained at 380 ℃ and 1×10-3 s-1. It is worth mentioned that high-strain-rate (＝1×10-2 s-1) superplasticity with elongation of 161.5% was achieved at 380 ℃. The mean grain sizes increase with temperature increasing and strain rate decreasing. The grain boundary sliding (GBS) is the dominant deformation mode of the superplasticity, which is verified by grains remaining equiaxial during deformation. Cavities form and grow up along grain boundary. And then the growth and connection of cavities results in the final failure. SEM morphology of the failure surface shows typical tenacity failure characteristics with quantities of uniform dimples.