The capacity of the power grid in China is increasing rapidly. Because of the reliability of the multi-infeed direct current (DC) system in the grid, power-system damping is relatively weak. Realizing isolated island operation of thermal power units with fast-cut-back (FCB) ability is considered to be the best solution for the quick restoration of the power grid. Taking a 1000-MW ultra-supercritical (USC) unit as an example, based on the field-test data of load rejection, this paper studies the dynamic responses of the turbine side during the FCB process. First, the maximum speed increase of the rotor during the FCB process with different loads is simulated using software. Second, the water levels of the deaerator and condenser during the FCB process are predicted with different loads using the same method. Third, the highest exhaust steam temperature of the high-pressure (HP) cylinder during the FCB process is calculated and predicted with different loads using finit-element modeling (FEM). Fourth, these key parameters are compared with the field FCB test data, and the comparison shows that the predicted results agree with the field-test data very well. Finally, the influence of the FCB test on the safety and life loss of the turbine side is analyzed.