Abstract: As water source of the Middle Route of South-to-North Water Transfer Project, the water quality of groundwater in Danjiangkou Reservoir is the key factor influencing the construction of South-to-North Water Transfer Project. In order to ensure the water transfer from Danjiangkou Reservoir to Beijing, the authors investigated the hydrochemical characters of groundwater, based on the sampling, statistics and related analysis of groundwater in Laoguanhe River Basin. The results show that the groundwater in Laoguanhe River Basin of Danjiangkou belongs to alkalescence and low mineral, and the dominant cation is Ca²⁺, while the dominant anion is HCO_3^-. Except for the NO_3^-, the main ions contents can reach the drinking water standard of China and the World Health Organization. The main hydrochemical types are Ca-Mg-HCO₃ and Ca-HCO₃ in both high-water and low-water periods, and the hydrochemistry process is mainly efflorescence-lixiviation. The NO_3^- content of groundwater exceeds 13% to 17%, and the Cl^- type groundwater may occur during high-water period. Season change barely influenced the spatial distribution of groundwater hydrochemical type in Laoguanhe River Basin, and the groundwater quality is mainly influenced by the human activities, such as agriculture, aquaculture, industrial residual waste and domestic sewage.