Abstract: The relation between viscosity of medium suspension and lower size limit of separation of cyclone in investigated theoretically based on result of analysis of velocity distribution of fluid and particles in cyclone and the force to which the rigid particles in solid-liquid flow in gravitational field are subjected,and through combined use of Euler field method and Lagrange tracking method. After making necessary assumptions and leaving out some omissible factors,it is concluded that the lower size limit of separation tends to drop to 1/√n of its original figure when viscosity of medium suspension is reduced down to 1/n of its original value. The soundness and confidence of the theorectical analysis have been borne out through simulation using Fluent software.