Experiment was undertaken for ex site-bioremediation of oil-contaminated soil which comes from Dagang oilfield, whether the exogenous microbes accelerate the rate of the bioremediation or not is studied and the impact factors on the biodegradation of the petroleum-contaminated soil were investigated too. The collected soil were divided into two groups then mixed, and the total petroleum hydrocarbon(TPH) was 8 416, 16 385 mg/kg respectively. The TPH of the soil was monitored during the treatment and the influence factors such as indigenous consortium, bio-stimulating, different exogenous microbe, bulk agent (sawdust), the varied initial TPH of the soil were investigated. The components changes of the bio-treated oil and the untreated oil were analyzed by GC-MS. The experiment was lasted for 300 d. When the water content of the soil is given, the exogenous microorganism can speed up the biodegradation of the petroleum greatly, and the reduction ratio of TPH reached to 79% with the combination of the exogenous microbe and bulk agent. The results of GC-MS show that the alkane can be better metabolized by microorganisms when their molecular weight are less than C ₂₈ and the microbes can biodegradate the polycyclic aromatic hydrocarbon efficiently.