Abstract: The traditional Riccati transfer matrix method exhitibs significant limitations in accurately reflecting the dynamic characteristics of steam turbine rotors under fault scenarios. Additionally, the identification of critical sections primarily relies on alternating stress amplitude, while neglecting the impact of non-zero mean stress, which may lead to deviations in fatigue damage estimation. An improved method was proposed, in which the steady-state motion parameters before the fault was taken as initial values.A three-point interpolation method was then employed to calculate the motion state of each shaft segment during the non-steady interval in real time, thereby capturing the true response when the fault was detected. Meanwhile, the fatigue damage was refined by incorporating the mean-stress effect based on Goodman's theory.A 600 MW steam turbine generator shafting was analyzed as an example to evaluate the fatigue damage caused by two types of resonance in the subsynchronous frequency range. Results show that the improved response calculation method significantly reduces errors caused by unsteady factors, and the identification of critical sections requires comprehensive consideration of the components and intensity of SSO excitation.