Abstract: Simulation experiments were carried out on a test rig for single-span rotor system with crack fault and the collected fault signals were analyzed by way of a 3-D waterfall spectra in combination with reassigned wavelet scalogram. The results showed that the combination can reveal the frequency structure of signal better, especially the relatively low-energy twofold frequency component which can be measured at half of the critical speed. It was found that an one-half order harmonic or subharmonic resonance comes up at twofold critical speed and with the crack deepened further, high order harmonics occur. These frequency characteristics features can be regarded as a reference to diagnosing the crack fault.