Abstract: The characteristics of pilot-scale membrane sequencing batch reactor (MSBR) treating synthetic wastewater were studied.The COD_Cr, NH₃-N and TN removal were 95.0%，96.3% and 38.0% respectively.The reason for low TN removal was lacking agitated equipment. During static phase, most sludge settled to the bottom of reactor (SV<40%),and consequently the denitrifying bacteria could not contact with NO_x-N completely. The trans-membrane pressure (TMP) rise rate was 1.032 kPa&#8226;m^-1. Molecular weight distribution of mixed liquid and membrane foulants were analyzed by the steric exclusion liquid chromatography method. It demonstrated that the accumulation of macromolecular matter(molecular weight＞10⁴) in reactor and membrane were 64.0% and 38.0% respectively. After physical and chemical washing, membrane specific flux was recovered by 73.4%. The concentration of polysaccharides deposited on the membrane was reduced to 30.4%. Polysaccharide was the important substance to cause irreversible fouling.