Abstract: Gas flow behavior in a rough-cut cyclone in FCC disengager was studied. DSM model provided by CFX 5.5 was adopted to predict the gas flow field, and simulation results were compared with experimental data measured by means of five-hole probe. The comparison verified the good power of DSM model. The simulation and experimental results both revealed great differences between a rough-cut cyclone and a traditional one in terms of flow field.There existed reversed flow in the exit tube and dipleg,which could get to the separation space and strongly disturb the flow field. On the basis of the flow field, scalar transport equation was used to compute the gas residence time distribution. Simulation results of gas tracing showed that residence time distribution of the gas from the inlet and out of the dipleg and exit tube assumed logarithmic normal distribution. Gas residence time of the reversed flow took on a double-peak profile. Average gas residence time was increased by 5%—10% approximately for the existence of reversed flow in the exit tube. Decrease of diameter of the dipleg and existence of a hopper could increase mass flux out of the exit tube as well as decrease the reversed flow distinctly, thus the average gas residence time was decreased significantly.