Abstract: The forced double-frequency resonance of a large hydro-generator unit is studied. The resonance is composed of torsional resonance of whole rotor system and 100 Hz resonance excited by inverted-sequence electricity under short-circle between two phases. By applying energy method, the torsional resonance equations of the rotor system are derived. The study results show that compared with the vibrating amplitudes of single-resonance, double-frequency resonance is much larger, the resonance area gets wider and dynamic shear stresses increase greatly.