A cellular automata (CA) model for activated sludge treating biological wastewater was built, and the dynamics of biomass and BOD were studied. A new method estimating the return sludge ratio was provided by the CA model. The results show that the time for the BOD to reach the wastewater quality standard in the Moore neighbor model is shorter than for the Von.Neumann model, and that the concentration fluctuation of microbes in different growth stages is characterized by a time lag. The initial concentration of activated sludge has no influence on biomass and BOD in stable phase. The cellular automata model of activated sludge treating biological wastewater can provide theoretical evidence for sewage treatment.