Abstract: Based on the averaging method in the non-linear vibration theory and combined, with the mechanical energy conception, the energy method for the problem of multiple resonance in non-linear vibration of elastic shaft system has been found. Using this method we have researched, among others, the two fold resonance on rotor shaft system of generator. Its proved that the energy method is applicable and simple. Some new phenomena of two fold re-sonance are revealed. The theory is verified by experiment.