The surface temperature field change of AZ31B magnesium alloy was measured by infrared thermography during fatigue testing. Fatigue life of an extruded AZ31B alloy was predicted using different methods. Results show that the temperature evolution mainly undergoes five stages when the cycle loading is higher than the fatigue strength: temperature rising, the steep decrease, the constant value on temperature, abrupt temperature rise before specimen fracturing and final temperature drop. According to the temperature field change law, two methods were proposed to predict the fatigue life. The fatigue lives are obtained according to the temperature rise characteristics (?TM) in the course of fatigue test. The curve of ?TM-Nf shows that ?TM=3.89 oC is the maximum temperature increment, i.e. failure will occur when ?TM is more than 3.89 oC, and compared with the measured 3.68 oC, the error is smaller, 5.7%. Energy method is used to determine the fatigue life and draws the S-N curve. Fatigue strength is 99.3 MPa from the curve, and the error is 0.5 % compared with the measured result 99.8 MPa. The method of infrared thermography is convenient and time saving for evaluating the fatigue life of magnesium alloy.