In this paper, the flow stress curves during thermal deformation of AZ61B magnesium alloy have been measured by isothermal compression tests on Gleeble-1500D thermal simulation instrument, and effects of deformation temperature and strain rate on flow stress were analyzed. The thermal deformation activation energy of AZ61B alloy could be work out at 185.64 KJ/mol by means of empiric dynamic equation of elevated temperature deformation. The softening mechanism of AZ61B alloy under the testing condition of thermal deformation has been identified to be dynamic recrystallization.