The characteristic of heat production and transfer were discussed, and the connection between temperature distribution and stress distribution was analyzed. An energy model based on the energy theoretical model in the fatigue process was established to predict the fatigue life of the AZ31B magnesium specimens loaded the three-point-bending. The development of the microstructure of tensile region was analyzed. The prediction model was justified by the qualitative analysis of microstructure evolution. The temperature-evolution mechanism in the tensile region and the relation between the temperature rise and the twinning were discussed.