The precessing vortex core (PVC) is evidently responsive to the escape of small particles. In this study, a computational fluid dynamics simulation was applied to the gas flow inside a cyclone separator to investigate the PVC phenomenon, particularly to observe the deviation of the vortex core away from the separator geometric center. The results reveal that the interaction between the gas and particles evidently contributes to the PVC unsteadiness. Consequently, a distinct deviation of the vortex core reappeared in the working cyclone with symmetric inlets, while it was too weak to be observed during the pure gas cyclone flow. However, an apex cone at the dust outlet evidently weakened the vortex core motion deviation and decelerated its rotation in the gas-particle flow within the working cyclone. Owing to the radial confluence flow from the outer to inner vortex over the separation space, the upward inner flow swung unsteadily. If >13% of mechanical energy within the downward gas flow was injected into the upward inner vortex with a radial confluence flow, the vortex core deviated from the separator geometric center, resulting in evident vortex core deviation. (C) 2019 Elsevier B.V. All rights reserved.