Superplastic deformation mechanism of AZ31B magnesium alloy sheet was investigated. Maximum elongation of 216%and strain rate sensitivity of 0.36 were obtained at 723 K and a strain rate of 1×10-3s-1. It is found that dynamic recrystallization occurs at the early deformation stage, and that grain boundary sliding is the dominant deformation mode of superplastic AZ31B sheet,whose fracture is due to the growth and interlinkage of cavities nucleated at grain boundary. A cavity growth model was established, and damage characteristic parameters as well as the critical value of damage variable were identified so as to provide a theoretical ground on which the plastic forming technology of magnesium alloy sheet can be optimized.