Abstract: Al-Mn-Mg alloy was compressed at elevated temperature by dynamic thermal/mechanical simulation tester and the flow stress during hot deformation was studied. The Arrhenius equation was used to calculate the thermally-activated energy at peak and steady states of flow stress curves, constitutive equations were established, and the calculated results of the constitutive equation were compared with the experimental results. The results show that dynamic softening character of the alloy was remarkable because dynamic restore and dynamic recrystallization occurred during hot deformation, however, the flow stress curves displayed uncontinuous dynamic recrystallization under the conditions of high strain rate and elecated temperature. The thermally-activated energy at peak and steady states were 164.54 kJ·mol-1 and 187.26 kJ·mol-1, respectively. The calculated values of flow stress by constitutive equation were coincide with experimental ones.