The interdependent relationship of fatigue stress and span was regarded as a grey system using grey system theory and method. The stress level of fatigue test was uniformly divided into many grades and the whiting value of safe span was found out. A smooth discrete function was formed based on one accumulation of safe span value series. Therefore, the model of grey system was set up and the differential equation of grey system was solved. The accuracy of model was improved by revising the residual model. The safe span value at every stress level could be calculated using the reduction model of the grey system. It provided a new way for the safe span calculation of mechanical parts, which would be of great application value.