A model test rig of single-span rotor system was set up. By loosing tap bolts at one side of bearing housing to weaken elastic stiffness of supporting structure, the pedestal looseness faults existing in rotating machinery were simulated. The collected vibration signals with looseness faults at vertical direction have been analyzed by the 3-D waterfall spectrum, wavelet scalogram and trajectory. The analytical results showed that the chaotic low frequency components occurred near the first critical speed and the complicated nonlinear phenomena appeared, such as quasiperiodic motions and bifurcations etc. When the rotational speed exceeded double critical speed, the low frequency components tended towards stability.