Abstract: Improved rainflow counting method and modified linear cumulative fatigue damage theory are used to calculate life loss in the dangerous area on twophase shortcircuit working condition. It not only meets the requirement of realtime monitoring in the power plant but also makes the result more accurate by this way. Due to the limitations of taking maximum torsional stress as the index of torsional vibration assessment for axis system, this paper proposes fatigue life loss to reflect the safety of dangerous area. An example is given and the curve of fatigue life loss with time under twophase shortcircuit working condition is drawn finally.基金项目:新世纪优秀人才资助计划(NCET080769)