Objectives: Sandu River basin in the upper reaches of the Weihe River is a well-known distribution area of brackish groundwater in the Loess Plateau. The social and economic development of the region is constrained by the issue of “pollution induced water shortage”. Make an intensive study of the formation mechanisms in brackish groundwater is of great significant to the rational development and utilization of groundwater resources, as well as ensuring drinking water safety. Methods: In this paper, it takes the water chemical detection data collected from 26 sites in the middle and southern parts of the San Du River basin as the research foundation. The hydrochemical analysis, correlation analysis, ion ratio analysis and mineral saturation index analysis were investigated to identify the hydrochemical characteristics and ion sources of groundwater in the Sandu River Basin, and discuss the formation mechanism of brackish groundwater. Results: The results show that phreatic water of river valley was generally brackish water, the average values of total dissolved solids ( TDS ) and SO42- are 5599.7 mg/L and 1973.1 mg/L respectively in the research area. The hydrochemical types are mainly HCO3-Na, HCO3?SO4-Na, SO4?Cl-Na?Mg, Cl?SO4-Na?Mg, and SO4?Cl-Na. In addition, the distribution of brackish water is closely correlated with the meizoseismal area of “Tongwei earthquake” in history. The phreatic water of loess is a recharge area for regional groundwater in the study area, with a short and intense circulation pathway, and better water quality than the phreatic water of river valley. The TDS are 839.6~1237.6mg/L. The hydrochemical types are mainly Cl?SO4?HCO3-Na, HCO3?Cl-Na and HCO3?SO4?Cl-Na. Conclusions: The hydrochemical compositions of groundwater mainly come from the weathering and dissolution of gypsum and halite; Lixiviation is the main controlling factor for the formation of brackish goundwater in the study area, followed by cation exchange, but evaporation and human activities have a slight impact; The increase in soluble salts in valley deposits and changes in the conditions of groundwater runoff caused by the Tongwei earthquake is the root cause.