Control of dissolved oxygen(DO) in oxidation ditch system involves several variants with great time lag

which makes it ineffective to adopt PID approach or simple fuzzy technology for DO control.Fuzzy reasoning strategy has been adopted for controlling DO density in the oxidation ditch system and a DO fuzzy controller has been designed.Simulation experiment on the controller proves that the controller based on fuzzy reasoning is of small overshoot and quick response.The controller can be used for the automatic control of sewage treatment system.