Abstract: Objective: To explore the transient thermal mechanical parameter decoupling mechanism and fatigue damage accumulation evolution law of steam turbine rotor under multi axis non proportional loading path during frequent start-up and shutdown of thermal power units, and to solve the engineering problem that traditional maintenance strategies cannot accurately evaluate the remaining life. Method: Based on the flow modification project, a finite element recursive model of the rotor"s unsteady temperature field and thermo mechanical coupled stress field is constructed. The critical plane method is introduced to characterize the equivalent stress amplitude of the multi axis stress state, and the Miner linear accumulation rule is used to achieve the successive superposition of the damage degree of each start stop cycle. Result: Clarify the differences in damage increment and nonlinear stage characteristics of damage evolution in the rotor danger zone under different thermal start stop cycles, and verify the effectiveness of flow modification in reducing peak thermal stress. Conclusion: Establishing a quantitative evaluation method for rotor fatigue damage provides a theoretical basis for life prediction and maintenance decision-making of peak shaving units based on damage status.