In order to obtain the information of the high-performance cyclone classifier, three-dimensional numerical simulation and experimental studies are conducted on the fluid flow and particle trajectory in a cyclone classifier with and without blow-down. By use of the blow-down method, the overall separation efficiency of a cyclone is higher than the case without blow-down. From the results of simulation, the reason for this is found to be the fact that downward velocity components and circumferential velocity components near the conical wall are stronger in the case of blow-down compared to the case without blow-down. It is also found by the simulation that regardless of the use of blow-down method, as the initial position of particles at the cyclone entrance is lower, the partial separation efficiency becomes higher. It is also made clear by comparing the calculated partial separation efficiency and experimental results that in the case of blow-down method, the repulsion probability of particles near the wall becomes lower than the case without blow-down.