Abstract: The three-dimensional unsteady flow field in a cyclone separator was simulatedThe results showed that the precessing vortex core (PVC) phenomenon existed in all axial positions of the cyclone separatorThe PVC phenomenon and motion of precessional center of the core was described in detail.The amplitude and frequency of velocity fluctuation in the PVC region at different axial positions was quantitatively analysed,and the scope of influence of PVC and the nature of the velocity fluctuation were discussed.Furthermore,the strength of PVC was detemined by analyzing the motion frequency of precessional centerThe simulation results agreed with the experimental results of hot wire anemometer and particle image velocimetry (PIV ).The influence of PVC on separation efficiency and pressure drop was analysed.