Abstract: In the present paper, numerical simulation on separating process of cyclone separator used in the steam generator is performed under conditions of different steam velocity based on two-fluid model and Reynolds Stress Model. The effect of steam velocity on separation process of droplets is analyzed. The simulation results show that the effect of steam velocity on separation property is related with droplets diameter. The effect of steam velocity is significant and separation efficiency increases with the increasing of steam velocity only when the droplets diameter is in a certain range. It is also found that liquid stack will happen on the wall of cyclone separator when steam velocity is small.