Abstract: The choice of culture medium of microbial isolation and measuring technologies including PCR，ERIC，DGGE，TGGE，FISH and T-RFLP are introduced briefly.Then，the microbial population characterizations in the activated sludge，i.e.microbial diversity， microbial community structure stability and its function stability are stated.Finally，it is pointed out that the microbial diversity in the activated sludge is the base of its community structure stability which is the precondition of its function stability being the guarantee of effective substrate degradation.Inreverse，the variety of substrate and environment factor will also affect the stability of community structure and function，and may make the population diversity change.The change of the microbial community structure stability doesn't keep in step with that of its function stability.