Abstract: In order to investigate the hydrochemical characteristics and impact mechanism of shallow groundwater in the Yangtze River Estuary, 22 shallow groundwater samples and 3 surface water samples on Chongming Island were collected and tested for 11 water chemical indicators as well as hydrogen and oxygen isotopes. With the combination of methods such as descriptive statistical analysis, Arc GIS spatial analysis, Piper diagram and ion proportionality coefficient determination, the hydrochemical types and ion spatial distribution in the study area were determined. Furthermore, the main factors and ion sources controlling the chemical evolution of the groundwater were discussed. The results show that the pH of shallow groundwater in Chongming Island is between 7.30 and 7.94, representing neutral hard water, and 36% of the groundwater is extremely hard water. The degree of groundwater mineralization in the reclamation area is relatively high. The groundwater hydrochemistry is relatively complex in the study area, including 13 types. The major type is HCO₃-Ca water, accounting for 27% of water samples, and is mainly distributed in the middle and southwest area of Chongming Island. From the west to the east, the anions transition from HCO₃^- to Cl^-, and the cations transition from Ca²⁺ to Na⁺. In the reclamation area, the groundwater chemical is mainly Cl·HCO₃-Na. Therefore, water-rock interaction, cation exchange process and human activities are the main factors affecting the chemical characteristics of shallow groundwater in Chongming Island. There is a hydraulic connection between surface water and groundwater, and recharge of water in the area mainly originates from atmospheric precipitation.