Abstract: In order to deeply study the hydrochemical characteristics and formation mechanism of shallow groundwater in the plain area south of Beijing-Tianjin, groundwater investigation, water sample collection and analysis were carried out in Shijiazhuang, Hengshui and Cangzhou in the plain area south of Beijing-Tianjin, and groundwater chemical characteristics and the mechanism of water-rock interaction were analyzed by using a combination of Gibbs model simulation, hydrochemical analysis methods, major ion ratio analysis, and saturation index analysis by calculating with PHREEQC software. The chemical characteristics of groundwater and the mechanism of water-rock interaction were analyzed by using Gibbs model simulation. The results show that along the direction of groundwater flow, the content of TDS, K⁺, Na⁺, Mg²⁺, Cl^-, SO₄^2- gradually increases, and Ca²⁺, HCO₃^- gradually decreases. Groundwater chemistry type shifted from HCO₃-Ca type to Cl-Na type evolution; the mechanism of groundwater hydrochemistry in the study area, in the plain area of pre-mountain alluvial flood fan, the mechanism of groundwater hydrochemistry was dominated by hydrological action, and the dissolution of carbonate and gypsum mainly took place, while in the central plain area, the hydrochemistry was controlled by the joint control of hydrological action, sedimentation, and evaporation and crystallization.