Abstract: This article discussed the chemical and mineral composition of the abnormal substances discharged from drainage well in Bao-zhu-si dam base. The detailed analysis came to the following parts: （1） the origin of the discharged groundwater. （2） the characteristics of the composition of the discharged groundwater. （3） the chemical and mineral composition of dam base rock and construction materials. （4） the process of the formation of the abnormal substances. Results showed that the abnormal substances are formed by interactions between groundwater and dam base rock as well as construction materials of the dam. Among these the precipitation of quarts, calcite, dolomite, albite, plagioclase, the dissolution of hydroxy calcium oxide and gypsum as well as the deoxidization of SO 2- 4 to H 2S by organic materials are the most important reactions.