Deformation mechanism of magnesium alloys is complex and prone to be affected by initial process states and deformation conditions of materials during the thermal process, therefore, exhibits different stress-strain relationships. Stress-strain curves of casting and wrought magnesium alloys AZ31B were obtained by Gleeble-1500, and the constitutive models of magnesium alloys under two different initial process states were constructed based on the Arrhenius hyperbolic sine function, the effect of initial process states on stress-strain curves and deformation mechanism of magnesium alloy were analyzed. Experimental results show that wrought magnesium alloy appears shear fracture along 45° direction due to deformation texture and amounts of twins at low temperature when the strain rate is greater than 0.1s_^-1. However, The deformation mechanisms of casting and wrought magnesium alloys AZ31B keep the same at high temperatures and low strain rates, therefore, the stress-strain curves of them are basically similar. The hardening index n and activation energy Q of wrought magnesium alloys are smaller than casting magnesium alloys.