Groundwater is an important resource for alleviating global water scarcity. Therefore, understanding its hydrochemical characteristics and mechanisms is of great significance for groundwater management. A total of 210 groundwater samples from shallow monitoring wells in the plain area of Beijing were collected in this study. Statistical analysis, Piper diagram, spatial interpolation, Pearson correlation analysis, and the ion ratio method were used to study the hydrochemical characteristics, spatial distribution, and formation mechanism of groundwater in the plain area of Beijing. The results showed that ① The groundwater of the six groundwater systems in the plain area was mainly HCO