An integrated anaerobe/aerobic membrane bioreactor (A/O MBR) was built and used to treat urban sewage water. The characters of mixture was researched and analyzed. Because the concentration of active sludge in MBR is high, the load of the sludge during the function is really low. When the active sludge functioned under low load condition, because of the low concentration of nutrition the metabolism of microbial and extracellular polymeric substances (EPS) increased and the sludge settling performance turned bad. Result showed that the sludge volume index (SVI) of the MBR high than 100 all the time, and the microbe is in micro expansion condition. Analyzed by scanning electron microscopy, it is indicated that the reason of micro expansion of the sludge is high concentration of micelles bacteria made the sludge connected and weaken the sludge settling performance. The microbial diversity of the mixture was detected by PCR-DGGE, and the results showed that there were different kinds of microbe in the mixture under different working Conditions.