In order to prevent membrane fouling and improve the elimination effect of phosphorus,polymeric aluminum and fly-ash were fed to membrane bioreactor to change the property of wastewater treatment;meanwhile active sludge was analyzed with X-ray diffraction and infrared spectrum methods,and hollow membrane was probed with SEM to investigate the mechanism of membrane fouling.The results show that the property of active sludge and bio-membrane structure were changed by adding polymeric aluminum,especially the removal rate of phosphorous could reach 85% and the membrane fouling was reduced effectively.Moreover no effect was observed when ash-fly was fed into the membrane bioreactor.