Abstract: A novel method was proposed to realtimely estimate torsional stress in turbo-generator shafts caused by frequent subsynchronous oscillation (SSO) with low amplitudes in high-voltage direct-current (HVDC) power transmission systems, with which the torque in each weak cross section of shafts can be calculated by combining the torsional angle signal and 3-phase generator voltage and current signals collected in real time with torsional vibration modes and torsional rigidity in each weak position, and subsequently real-time torsional stress can be obtained based on the weak section structure characteristics and stress concentration factor. The method was used to estimate the torsional stress history of shafts in a 1 000 MW turbo-generator set, which had suffered an actual SSO with low amplitudes caused by HVDC. Results show that the proposed real-time calculation method for torsional stress is simple, accurate and reliable without complicated mathematical calculation, which is easy to implement via computer.