Abstract: In order to study the growth of the bacterium,the experiments were designed to enrich phosphate accumulating organisms(PAOs) under alternate anaerobic/aerobic conditions which fed acetate/propionate in synthetic water.Molecular biology techniques of FISH and PCR-DGGE were used to qualitatively and quantitatively analyze bacterial population of EBPR.DGGE results showed that the bacterial population had undergone significant changes which some indices,including Shannon index(H′),Margalef index(D_Mg) and the number of bands(R) had similar fluctuations,and had the highest value at the end of phase Ⅱ,which had decreased in the stable operation of the phase(phase Ⅲ).the dominance index(d) had increased in the phase Ⅲ.Cluster dendrogram analysis results showed that the bacterial population had higher similarity in the phase Ⅲ.FISH results showed that,PAOs and GAOs had the similar increased trends in the start-up phase and increase load phase,and accounted for 41% and 39% of total biomass.PAOs became dominance with compared to GAOs(<6%) and accounted for 89% of total biomass in the phase Ⅲ.