Abstract: The clearance energyofmagnetic field and electromagnetic stiffness of a large hydrogenerator unit are obtained, the formula for solving the whole torsional natural frequency of the hydrogenerator rotors is derived, and the zero frequency of torsional vibration under the rated running condition is calculated The change of zero frequency with the exciting current and the active power, and the magnitude-frequency character of resonance when zero frequency approximates to the working frequency, are analyzed.