A numerical simulation of superplastic backward extrusion of a magnesium alloy part is presented in this paper. In fact, the simulated superplastic forming is not a pure superplastic forming because of the billet with coarse cylindrical grains. The forming may become a pure superplastic forming only after dynamic recrystallization and grain refinement appear and the grain boundary sliding has been the main deformation mechanism. In order to simulate the special forming process, a constitutive relation considering dynamic recrystallization and the multiform deform mechanism and the parameter identification of the constitutive relattion are studied. The program for simulation is able to predict the grain refinement and the transform between different deformation mechanisms. Finally the calculated results on the grain size and dynamic rerystallization are presented. A comparison between the calculated and the experimental results shows there is a good agreement between calculated results and experimental results.