The vibration characteristics of the bearing rotor system was investigated in a single span four disc structure rotor test bed supported by hydrostatic gas bearing.Using the vibration test and analysis methods,such as the orbit of shaft center,time domain analysis,spectrogram etc.,the nonlinear dynamics phenomena,such as half speed whirl,double low frequency,sub synchronous resonance etc.were presented.The change of the half speed whirl,double low frequency,sub synchronous resonance with the rotating speed was analyzed,gave the qualitative analysis for power frequency and low frequency energy conversion relations,the amplitude of low frequency increased resulting in the speed drop.Higher rate in speeding process can avoid low frequency vibrations; lower rates in slowing process can also cause half speed whirl and double low frequency vibration phenomenon.