The asymmetry characteristics of the gas-phase flow field in a volute cyclone were analyzed with software package FLUENT 6.1 and its asymmetry mechanism was investigated based on the simulated results.Because of the asymmetry of the inlet structure and the unstable characteristics of the swirl flow，the spinning center of the gas flow deviated from the geometrical center of the cyclone，which resulted in the asymmetry of the flow field and the change of the three dimensional velocity components.Based on the Rayleigh criterion，the unstable characteristics of the swirl flow in the cyclone were intrinsic.The instability decreased with increasing swirl number，which weakened the degree of asymmetry of flow field.The swirl number did not vary with inlet velocity，which indicated that the inlet velocity did not affect the asymmetry of flow field.However，the swirl number increased with increasing ratio of cyclone body cross section area to the inlet area KA and decreasing ratio of the internal diameter of the exit tube to the cyclone body diameter de，which weakened the degree of asymmetry of flow field.The degree of asymmetry could be evaluated by the tangential angular momentum.