Abstract: Flow behavior of fluid and suspension of crystals of ammonium sulphate is the key to designing an industrial reactor in producing ammonium sulphate. A just suspending agitating speed, 100 r/min, was computed based on the crystal size distribution（CSD）of ammonium sulphate from experiments. Computational fluid dynamics（CFD）simulations were performed under 100 r/min for a developed industrial continuous crystallizer. The flow field of single phase was observed and compared with that under 200 r/min. Then the Lagrangian dispersed phase model was employed to trace the trajectories of crystal particles of deferent size in order to understand crystals' behavior inside the reactor under 100 r/min. It is concluded from computing results that the developed crystallizer of ammonium sulphate is reasonable.