To gain a better understanding of the mechanism and technology of microbial enhanced oil recovery, microbial community structure and diversity of reservoirs after polymer flooding in Daqing oil field at the Earlier Stage of microbial profile modification were studied. 16S rDNA gene clone library was used to assess the structure and diversity of microbial community. The results showed that the dominant microbes of the earlier stage of microbial profile modification are uncultured bacterium, comprising 88.6% of library clones. The cultured strains are composed of Epsilonproteobacteria(5.7%) , Gammaproteobacteria(4.7%) and Firmicutes (1%). Among the Epsilonproteobacteria, Sulfuricurvum accounts for 4.7% of the cultured strains of library clones and Arcobacter accounts for 1%. Besides, the dominant communities also include Pseudomonas and Moorella.