It is important to study system vibration characteristics for improving product quality and ensuring safety operation of units. In this paper, some dynamics of rotor systems are studied in large centrifugal compressor. The models of diaphragm coupling and tilting pad bearing are established by appropriate simplification. The finite element model of rotors with bearing and coupling is set up by taking interrelation of these subsystems into account. The unbalance responses of journal are computed based on the relative standard. The effect of bearing parameters (length-diameter ratio, clearance ratio and lubricating oil number) to journal vibration of rotor systems is analyzed. Finally test results and theoretical results are compared and errors are analyzed. The research results will provide theoretical basis for improving the bearing design of compressor rotor systems.