The risk of sub-synchronous oscillation（SSO） increases as increasing numbers of power electronic devices in power networks are being used to improve the systems' transmission capacity. The possible SSO phenomenon in power networks in China is analyzed based on the study of a 600 MW turbine generator shafting. The torsional vibration is analyzed based on the coupled system between units and the network; then the fatigue damage is studied based on the stress analysis and fatigue cumulation. The network disturbance amplitude and frequency are studied because they have significant impacts on shafting fatigue damage. The results can provide useful information in support of the unit's life cycle management and efficient evaluation of the permissible value of network disturbances.