Abstract: A model of hydrochemical equilibrium reactions is proposed based on the principle of equilibrium of complex actions in the solution, and used to calculate the concentrations and activities of different mineral species in groundwater. The index of mineral saturations related to the groundwater are then determined. So the tendency of dissolution and precipitation of minerals in the hydrorock reactions can be studied. This method was applied to study the basalt in Xuyi county, Jiangsu province and discuss the reactions between groundwater and basalt.