Abstract: The diffuser has an important influence on the performance of the compressor. In order to study the influences of the diffuser on unsteady characteristics of the centrifugal compressor, experiment and numerical simulation under three different diffuser conditions are carried out for a small-sized centrifugal compressor. The results show that under different diffuser conditions, the amplitude of pipeline pressure fluctuation during surge process is similar, but the surge frequency is quite different. Under certain conditions, the centrifugal compressor can achieve the same surge frequency when matched with a vaneless diffuser and a vane diffuser. The flow rate has less influence on the surge frequency, but has a great influence on the pressure amplitude. In the surge cycle, the pressure growth stage accounts for a higher proportion in the period of outlet pipeline pressure change, and matching the vane diffuser will increase this proportion.