Abstract: High-cycle fatigue property of AZ31B magnesium alloy plats at room temperature were studied, and the temperature changes on the surface of samples was tested by infrared thermographic technology. The fatigue fracture position and fatigue limit of the alloy was confirmed according to the relationship between temperature and stress during fatigue test and the temperature distribution difference on sample surface. The results show that the fatigue fracture position could be predicted according to the change of the temperature on alloy surface, and the fatigue limit tested by the method was 107.5 MPa, the value was agreement with the fatigue limit tested by conventional fatigue test. The temperature of magnesium alloy increased to a certain temperature, and then decreased to room temperature at the earlier stage of loading, and the temperature raised quickly before fracture.