Attentions have been paid to Hg anomalies in soil in town areas and their surroundings nowadays. However, no clear conclusion has been drawn on the ecological effects induced by Hg anomalies at present for lacking of exact understanding of the mechanism of the cause. The results of this paper showed that there was causal relationship between Hg anomalies and the existence of mineral cinnabar, which was formed by the crystallized compound of HgS. And this compound was combined by the anion S2- with the cation of Hg2 which was derived from all kinds of environmental pollution. So heavy metal Hg was fixed by the form of mineral cinnabar and resulted in Hg anomalies in soil. The results provided scientific evidence for evaluation and forecast of the effects of eco-geochemistry induced by Hg anomalies.