The microstructure, the thermal tensile behaviors and the gas bulging forming formability of AZ31B Magnesium Alloy were investigated by thermal uniaxial tensile，gas bulging forming，optical microstructural observation and SEM observation. The results show that in the condition of 425 ℃, with a deformation rate of 1.0×10-3 ~ 6.6×10-5 s-1, a flow stress of 4~12 MPa, and an elongation of 200 %~327 %, the extrusion + hot/cold rolled magnesium alloy shows a good superplasticity. The AZ31B magnesium alloy shows a better formability at 425 ℃, 1.0×10-3 s-1 with a bulging height of more than 24 mm and H/r>0.80