By the test of rotational bending for bar of 45 steel with V notch in low cycle fatigue, the bar’s fatigue life is studied under strain-controlled condition. The characteristics of several kinds of specimens' crack propagations are analyzed. The accumulative effect of crack propagation is discussed to find the appropriate load for quick fracture. Based on the theory of continuum medium damage mechanics, the damage evolution model in low cycle fatigue is obtained. The predicted curves and strain-life curves agree with experimental data for medium carbon steel well in this work. These results are very important for the life estimation of medium carbon steel.