Abstract: The numerical simulation analysis of rectangular settling tanks was carried out in wastewater treatment plants ( WWTPs) by using the Fluent15. 0． The RNG k － ε and eulerian two-fluid models were selected． The reliability of the selected model was verified by the experimental results of Imam in the University of Windsor in Canada． Two indicators were selected including the concentration of effluent suspended solids and the proportion of density flow． Three factors were analyzed including the size of sludge particle，the surface overflow rate and the proportion of baffle． The results showed that the effect of separation between sludge and water was influenced by the size of sludge particle． Moreover，the optimized particle size was 300 μm． The surface overflow rate influenced the concentration of effluent suspended solids intensively． The concentration of the effluent suspended solids and the proportion of density flow were increased by nonlinear with the increase of surface overflow rate． In addition，the optimized proportion of baffle was 60% in the rectangular settling tanks． At this proportion，the energy dissipation was increased，the proportion of density flow was decreased． Meanwhile，the concentration of effluent suspended solids was lower．