In this paper,with the object of maximum usefulvolume effieiency,the optimization of structures of downflow wastwaterdistributors in an anaerobic fluidized bed under the condition of sameexpansion height and same return flow rate is studied by means oforthogonal test and statistical analysis.The sequence of importance ofthe factors affecting the performance of the distributors is given andthe optimum structure of downflow wastewater distributors is obtained.