In the North-western loess Plateau,a new oil fidld with area of several hundred square kilometers has been found.It's exploitation will accelerate the rapid expansion of Chinese oil industry, yet its effects on ecological environment would be quite serious.Above all,the technologies used are all pollution-free advanced technologies.But there are some polluting sources.This oil field can produce 2.5×10⁴ tons of waste warer per year,3239×10⁴m³/a waste gases,2×10⁴ tons solid pollutants.The total of the oil fallliig upon the ground is 97.8-100 tons,the results are revealed in this paper,The oil field occupied land area is 1.3-2.0 Km², the content of soil oil is increased to 77.2mg/kg.and the agricul turaloutput has been decreased by about 0.8-2.7%.The result of the exploitation related to slope area is that the area of slope land over 15 degrees is reduced and that beiow 5 degrees is increased.So it is beneficial to protecting the soil and water.The assessed result shows that the effects of oil field exploitation on ecological environment were high in the region(Ⅱ),but low in regi(Ⅲ).We shall strengthen the conservation of the eco-environment in region(Ⅱ)in order to reduce soil pollution of the oil field as well as to outpiit of crops, and to improve the environmental quality of the improve the oil field.