The utilization of hydrogen is gaining increasing attention due to its high heating value and environmen-tally friendly combustion product.The supercritical water circulating fluidized bed reactor is a promising and potentially clean technology that can generate hydrogen from coal gasification.Cyclone is a vital part of the reactor which can separate incomplete decomposition of pulverized coal particles from mixed working fluid.This paper aims to gain in-depth understanding of the cyclone separation mechanisms under supercritical fluid by computational fluid dynamics (CFD).Although the amount of supercritical carbon dioxide in mixed working fluid is minor,it obviously influences the flow fields and separation efficiency of a cyclone.The simulation results suggest that both the decreasing content of supercriti-cal carbon dioxide and adding the extra dipleg cause the promoting performance of cyclones.Research findings could refine the design of supercritical fluid-solid cyclones.