It is important to study system vibration characteristics for improving product quality and insuring safety operation of machine. In this paper, dynamics of rotor systems are studied in a large centrifugal compressor. The models of subsystems, such as coupling and bearing, are established by appropriate simplification. The finite element model of rotors is set up by taking interrelation of these subsystems into account. The unbalance responses and system stability are analyzed for overall unit and single cylinder. The results show that the critical speed and journal amplitude at working speed in the overall unit are greater than those of low pressure cylinder. The conclusion will provide guidance to compressor rotor system design.