The method of measurement and accurate assessment of structural fatigue damage is a challenging issue. In the process of structural fatigue damage, material magnetic properties changes, and the magnetic measurement is simple and convenient. Hence, it is significant to detect structure fatigue damage using the change of material magnetic properties. Using Q235 steel as test sample, this work focuses the magnetic measurement method of structure fatigue damage based on hysteresis loop under cyclic tensile stress, in which the test platform is set up. The results show that the hysteresis loop changes when the fatigue damage is different. The coercive force Hc and remanence Br of the hysteresis loop are extracted as feature parameters. The analysis results indicate that the whole fatigue process can be roughly divided into two stages before the specimen failure. Hc and Br rise rapidly in the first stage, whose sensitivity of the fatigue damage are higher. Comparatively, Hc and Br change slowly in the second stage, whose sensitivity of the fatigue damage are lower. Finally, the quantitative relationships of Hc, Br and fatigue accumulation damage D are analyzed. The proposed method can provide quantitative evaluation of the structural fatigue damage and online monitoring technology.