Abstract: The moving bed biofilm reactor (MBBR)-membrane bioreactor (MBR) combined process was used for the treatment of domestic wastewater. The removal effect of the reactors on the pollutants was investigated, the microbial population changes and compositional diversity in the reactors were analyzed by denaturing gradient gel electrophoresis (DGGE), and the microorganisms were classified by using cloned library. The results showed that the NH_4^+-N and COD_Cr in MBBR-MBR could acquire satisfied removal efficiency under stable operation, which were 98% and 97%. DGGE fingerprint showed that during the operation of the reactor, the microorganism community structure evolved obviously and was closely related to the temperature change, gradually forming a low temperature stable population adapted to the sewage treatment. Compared with MBR, MBBR had richer microbial population, more complex system and stronger resistance. The results of cloned library showed that the dominant species of microorganisms in MBBR and MBR were quite different, but both belonged to Proteobacteria.