The flow rate of downstream of the gas in a cyclone separator is closely related to the performance of cyclone separator. The flow in the cyclone separator was numerically analyzed by STAR-CCM+,and the defects of traditional downstream calculation method was found.An improved method for calculating the flow rate of downstream of the cyclone separators was presented based on the simulation results, and the influence of each parameter on flow rate of downstream was analyzed by using the concept of self-defined dimensionless average stroke. The results show that the dimensionless average stroke is increased with the decrease of the inlet velocity, the increase of the inlet depth of the riser,and the decrease of the inlet width. With the decrease of the diameter of the riser, the dimensionless average route is decreased first,and then increased gradually.