Abstract: Due to the bad working conditions, fatigue damage becomes the main form of drillstring failure. With the increasing understandings of drillstring work environment, theory and practice proves that fatigue damage is a cumulative process, which is related to the flexural-torsional alternate bending stress acted on drillstring. The mechanical model of whirled drill-string is established, and the stress especially the alterative bending stress of the drill-string is analyzed in this paper. At the crack tip, the plastic deformation and the plastic zone correction is taken into account to broaden the applied scope of the liner-elastic mechanics in this paper. Moreover, the Forman fatigue life prediction model is used to predict the drillstring fatigue life, which makes the result better. Fatigue damage makes the safety coefficient of drilling string higher.