Experimental study is performed on the probabilistic cyclic stress-strain responses of grade B cast steel, which is utilized in the manufacture of Chinese K6 type bogie. Fatigue test is carried out by an incremental strain-controlled method with 8 strain levels. Analysis of test data and observation on fatigue fracture surfaces of test specimens indicate that difference and evolution of local conditions are basic causes of the random fatigue damage behavior. So it is verified again that the random character of stress-strain responses is an intrinsic fatigue behavior of engineering materials. Taking the scattering regularity and the size of specimens into account, cyclic constitutive models of present test data with arbitrary probability and confidence are obtained. The analysis reveals that these models can reflect the random responses of material correctly.