To study the superplasticity of AZ31B magnesium alloy, hot compression tests were performed in forming temperature range from 280°C to 440°C and strain rate range from 0.001s-1 to 0.1s-1. The influence of deformation strain rate and forming temperature on flow stress was also analyzed detailed. It was shown that the flow stress of AZ31B was very sensitive to formpng temperature and stain rate, and was decreased with deformation temperature increasing, and was increased with stain rate increasing. However, no significant change of flow stress was observed at the temperature of 440°C and the strain rate below 0.01s-1. The activation energy of AZ31B in superplastic deformation was 141.6KJ•mol-1 and its constitutive equation was established also.