This study found out hydrochemical characteristics and genesis mechanism of groundwater in Dongting Lake Plain of the groundwater. The study collected 693 groups of groundwater and surface water samples collected for 6 years, and adopted several mathematical statistics methods (i.e., Piper diagram, Gibbs diagram, chlor-alkali indices and ion ratio analysis) to analyze the hydrochemical evolution of groundwater based on the chemical data. The shallow groundwater and deep groundwater in the study area were mainly SO4 type, \( {HCO}_3^{-} \) ·SO4 type and HCO3 type water. The groundwater composition is mainly determined by water-rock interaction. Gypsum and mirabilite are the main sources of sulfate in groundwater, and dolomite and calcite are the main sources of carbonate. Ca2+ and Mg2+ mainly come from dissolution of dolomite, calcite and gypsum. Na+ is not only derived from rock salt, but also from the dissolution of albite and mirabilite and cation exchange in water and soil.