There are many advantages of magnesium alloy such as light mass, nonrattling and electromagnetic interference shield. Yet, its ductility was poor at room temperature and plastic forming was difficulty due to its close-packed hexagonal structure. In this paper, the superplasticity of magnesium alloy AZ91D was explored and the relationship between flow stress and true strain were achieved at different temperature by tensile tests. It was found that the flow stress of AZ91D as cast was less than 20MPa at 340-450°Cand superplastic deformation could be used to manufacture magnesium alloy parts. Utilized superplastic air bulking mold, AZ91D seamless pipes with variable cross-section were manufactured successfully based on process parameters being optimized. Moreover, the relationship between loading path of internal pressure and wall thickness was analyzed also.