Aiming at overcoming the low plasticity of magnesium alloy at room temperature,we researched viscous warm pressure bulging(VWPB) of AZ31B magnesium alloy based on the excellent thermal stability of viscous medium under the warm forming condition.The potential improvements of plastic deformation ability and forming quality of AZ31B magnesium alloy are expected with the aid of thermal characteristics of viscous medium.During bulging process the velocity field variation and pressure stress field distribution of viscous medium are observed at different temperatures through which the effect of temperature on the mechanical property of viscous medium and AZ31B magnesium alloy are analyzed.The results show that the formability of AZ31B magnesium alloy increases first and then decreases as the temperature increases and it is the best at 200 ℃.On the other hand,the viscous medium which can build non-uniform pressure stress field also exhibits a good flow property at elevated temperature,and it is helpful to improving the formability of AZ31B magnesium alloy.