A　cellular　automata　model　for　a　sequencing　batch　reactor　was　constructed　according　to　biological　mechanism　of　activated　sludge　processes.　Simulations　were　made　with　practical　parameters.　The　results　indicate　that　the　model　replicates　complex　evolvements;　of　microorganism　during　reaction　time,　and　well　describes　the　removal　of　biochemical　oxygen　demand　and　the　growth　of　activated　sludge.　The　model,　whose　validity　is　proved　by　Eckenfelder　formula,　simulates　activated　sludge　processes　visually,　and　exhibits　advantages　of　modeling　wastewater　treatment　bases　on　cellular　automata.　It　offers　a　basic　method　for　modeling　similar　complex　systems.　At　the　same　time,　the　model　depicts　the　characteristics　of　complex　system　such　as　diversity,　randomicity,　indeterminacy　and　strong　nonlinearity.