Centrifugal compressors are widely used to compressor gas in many different processes within the petroleum and chemical industry. Two vibration types of concern in industrial compressors are synchronous and subsynchronous vibration. The latter, more troubling type of vibration, occurs when non-conservative whirling forces(Cross-coupling) act to excite a lateral natural frequency, which lies below the running speed(subsynchronous vibration). In this paper, more attention is paid to impellers from a rotordynamic standpoint and a synthesis gas compress in ammonia plant is analyzed in order to diagnosis the root cause of vibration fault using the best available resources for seals and bearing. Logarithmic decrement predictions for the full rotor model consisting of all the stages and seals are conducted under different bearing structure. On this basis, a resolvent to improve the rotordynamic stability is presented. The field application result indicates that this method is effective.