The present work investigated the variations in the active characteristics of sludge during the operation period of an aeration membrane bioreactor, as well as their effects on the fouling of membrane module. Through discharging the excess sludge, the concentration of the mixed liquor was maintained at around 4 000 mg x L(-1) for 75 days, during which the characteristics of sludge were carefully tested every day to reflect its variation. Results indicated the dehydrogenase activity increased with the prolonging operation time. The increased activity of dehydrogenase had two effects on the operation of the bioreactor, one was the enhancement of pollutant removal by the microorganisms, while the other effect was the increase of extracellular polymeric substance (EPS), which led to a deteriorative effect on the membrane fouling. During the operation of the bioreactor, the sludge yield coefficient first increased, and then decreased to a certain degree with obvious decrease in particle size and increase in the amount of EPS, which also enhanced the quality of the effluent. At the same time, protozoon and metazoan were observed in the whole bioreactor at the late operational phase but relatively rare at the initial phase. Analysis of membrane fouling showed that the fouling rate of the membrane module increased during the late operational period, which was attributed to the deposition of fine sludge particles and EPS caused by the decreased particle size of the sludge and the increased amount of EPS.