Surge happens when the mass flow rate reduces to a certain value in the centrifugal compressor and pipe network system. Surge will cause fatal damage to the centrifugal compressor. So researchers need to capture the surge fluctuation in the compressor and pipe system by methods. These works can research more details of surge. This study takes an attempt to use the CFD method with a plenum model to investigate the surge fluctuation in a centrifugal compressor and pipe system. Using the three dimensional transient numerical simulation, authors obtain the low frequency fluctuation of the mass flow rate and the total pressure at the diffuser outlet over simulation time. The flow field in the impeller and the diffuser passage are also obtained in the simulation. Authors take more detail researches on the surge fluctuation by the flow field result. By changing the plenum volume and the compressor operating point, authors compare and analyze the surge fluctuation under different operating conditions. The changing plenum volume has great influence on the mass flowrate fluctuation and the total pressure fluctuation. Only rotating stall will occur when connecting a smaller capacity pipe network. But the rotating stall will lead to the surge of the system when connecting a larger capacity pipe network. The decreasing mass flow rate has little influence on the fluctuation frequency but the fluctuation amplitude increases. It is verified that the surge in the compressor system is much more serious when connecting a larger capacity pipe system or decreasing the centrifugal compressor operating mass flowrate.