Abstract: As different geological bodies have different background values of metallogenetic elements, the ore deposit may be not distributed in the range of geochemical anomaly, and hence the anomaly might only reflect the background of the rock. Based on 1:200000 geochemical surveying data obtained in Zhangjiakou area, the authors calculated the anomaly thresholds of Cu, Zn, V, Ti and Ni elements. The results show that the geochemical anomaly based on traditional statistics is determined by the distribution area of basalt. That is to say, the range of geochemical anomaly is controlled by the lithology. According to the background value of different lithologies, the authors computed contrast values in comparison with geochemical data. The results show that this calculating way is not affected by lithology. This method can effectively delineate geochemical anomalies in low background value area. The contrast value method has good prospecting indicator significance.