To explore the evolution law of the groundwater chemical characteristics in the loose porous aquifer of Tongzhou District, Nantong City, a comprehensive analysis was conducted on the groundwater chemical characteristics and the spatio-temporal evolution of the main ion components in Tongzhou District from 2016 to 2023 by using mathematical statistics, Piper trilinear diagram, Gibbs diagram and ion ratio methods. Research results show that the groundwater is generally weakly alkaline. The water chemical type of the first confined aquifer did not change significantly from 2016 to 2023, while that of the third confined aquifer gradually changed from HCO₃-Na type to Cl-Na type. The phreatic aquifer and the third confined aquifer are mainly affected by water-rock interaction, while the first confined aquifer is controlled by seawater intrusion, dissolution of stratum salts and evaporation concentration, presenting the characteristics of high salinity brackish water. Total dissolved solids content of groundwater is highly correlated with Cl- mass concentration, indicating that Cl- is the main ion affecting total dissolved solids in groundwater. During the process of groundwater level recovery, water-rock interaction in the first confined aquifer is strong, promoting the release of Cl- from the soil into the groundwater, which aggravates the salinization of this aquifer. Meanwhile, the brackish water from the upper aquifer seeps through and replenishes the third confined aquifer, leading to a light salinization trend in this aquifer as well.