Four predominant bacteria that can degrade petroleum efficiently were obtained from the sludge in the sewage of Yan’an oil refinery by adapting, purifying and screening processes. The degradation efficiencies of petroleum hydrocarbon of four strains and their mixed strains were studied by the method of shaking cultivation. The bioremediation experiments of oil-contaminated soil were conducted by using soybean, alfalfa and the mixed strains. The results indicated that petroleum hydrocarbon of four strains and mixed strains reduced by 78.19%, 74.36%, 62.17%, 54.36% and 83.73% respectively with the initial concentration of 10000 mg/L after 25 days. The petroleum concentrations in the experimental fields of soybean and alfalfa reduced by 46.83% and 41.27% respectively after 120 days of bioremediation. The introduced mixed microbes increased the biodegradation rate of two plants by 67.14% and 56.92% respectively. Soybean or alfalfa - soil microorganism- introduced mixed bacteria showed the significant effect in the combined bioremediation of oil-contaminated soil.