The subject in this study is Wangyao reservoir, whose sediment is divided into three cross section. On each cross section, four points are set vertically to research the petroleum pollution of every layer. The result shows that, Wang yao reservoir, the main water supply source of Yan'an, contains a relatively high level of petroleum in sediment. The first layer(0∼10cm) contains the highest, generally above 200mg/kg, and can cause secondary pollution in overlying water easily; the soil sample collected 50cm below was mainly deposited 2000 years ago. The pollution of this layer is apparently very high. This infers that petroleum pollution was in the most severe situation in Yan'an from 1997 to 2000 and had a serious influence; the petroleum pollutant in sediment is of higher contents when it is nearer the dam.